Continuity Nod

"Ben and I decided that we needed more strips and punchlines that only make sense to hardcore readers. Look forward to jokes so inaccessible even we, the authors, don't get them."

- Lewis's Notes, Terror Island, Strip #200

A continuity nod is a reference to some aspect in a series' past or an occurrence within the universe. The nod is not so much a major plot point, but more of a tilt of the hat to the fans that may know these little details. It is a sort of Shout-Out.

In remakes of older TV shows or movies, this may often manifest itself as a form of Stunt Casting. If characters reference something outside the universe itself but has special meaning some other way (such as an Alternate Continuity or adaptation), it's a Mythology Gag. If it's a joke about the actors of the parts it's Actor Allusion or Casting Gag. References towards something that's been written out (or Take Thats towards something that people wish were written out) are Discontinuity Nods. A nod that affects the plot in some way can be considered a Olafs Hammer.

Never Live It Down and Remember When You Blew Up a Sun? are Sub Tropes. Oblivious Mockery is also a subtrope when the viewer knows about the action being mocked.

Distinct from a Call Back, as it does not actually drive any plot development, or a Call Forward, as it isn't going to be a plot point later on. If a single scene presents an unusually large number of Continuity Nods, all at once, you may have fallen under a Continuity Cavalcade. If there are so many Continuity Nods they detract from the story itself, it's Continuity Porn.

'Remember, it's only a continuity nod if the events happen in the same continuity''. Otherwise it's a Mythology Gag.'''

Anime & Manga

 * Carl Macek had to invent several of these when he made the multifarious Robotech. For example, Yellow Dancer is a singer who emulates Minmei.
 * This happens from time to time in One Piece, primarily through Usopp. When Usopp was first introduced he would regale Ill Girl Kaya with tales featuring things like a goldfish so large its poop was island sized, or claim he was sneaking around hunting a mole monster. After the Little Garden arc, the Straw Hats encountered a goldfish exactly like the one Usopp mentioned. During the Alabasta arc, Usopp faced Miss Merry Christmas, who had the power to transform into a mole-woman. The former case was lampshaded by Usopp commenting that the goldfish seemed familiar to him for some reason.
 * Also the time when Zoro had to defeat Mr. 1, who had the ability to compose his body to the hardness of steel. To learn how to cut steel and gain one step closer to defeating his rival Dracule Mihawk, he defeated Mr. 1. He then asked Zoro as he fainted if he plans on cutting diamond next. Recently, Mihawk himself  tried to use his strongest sword slash to see the difference between the two, only to find it blocked by 3rd division commander Jozu,.
 * During the anime's adaptation of the various Marine officers gathering in Marineford, Jonathan, The Chessmaster commander from the G8 filler arc, is shown arriving as well, accompanied by his subordinates Drake and Gu.
 * When Zoro makes his reappearance in Chapter 599, he makes his entrance by slicing a boat in two and mocking its inhabitants amongst the wreckage. Perhaps he gained more than just the man's skills in that time.
 * During the second season of Code Geass, Lelouch is shown walking past a wall in the school that is covered in cross-like marks, the end of the wall has blank bricks and a brick with just one line on it. This is a reference to the first season when he used his Geass on a girl with the instruction "make a mark on this wall every day" to test how long its effects last.
 * In Chrono Crusade, when Chrono tries to explain who the Pursuers are, he notes "If you count Leiraje, this is only the second time we've seen one!" (Leiraje was a character that had shown up in the first volume and kidnapped Azmaria.) Azmaria squeaks out "Really??" and Rosette snorts "Geez, nobody remembers him."
 * In the fifth episode of Princess Tutu, a lamp is possessed by a heart shard when she believes that she's been abandoned by the family she used to serve, and everyone has forgotten her. Princess Tutu told her that she was sure someone else would find her and love her for her light and warmth again, and sure enough, Ahiru takes the lamp home with her at the end of the episode. It's often seen in her room in later episodes.
 * This happens a few times in Yu-Gi-Oh! GX and 5D's, in particular referencing the original series. In GX a number of times, they reference a couple of the more famous battles outside of climactic showdowns, provided they were based on the manga at least, and occasionally referenced or alluded to well-known events or cards, like in episodes 18 & 19 where Judai dueled against a copycat who had stolen Yugi's deck. This occurs far-less often in 5D's, aside from the fact it's in what we used to know as Domino City, though there are a few allusions to icons of the past on some of the newer cards.
 * The Speed Fusion card portrays Dark Magician and Flame Swordsman fusing into Dark Flare Knight.
 * This happened plenty of times within the original series as well. The Pharaoh walking through a valley of lost spirits and flashing back to a bunch of minor antagonists. The cameos that Doma's swordsmen received after the Waking the Dragons arc, along with everyone else in the last few episodes of the whole show. While not free of filler, the franchise has a truckload of continuity nods.
 * Ayukawa, the school nurse in the GX anime, makes an appearance in the GX manga observing a student's coma and noting a similarity between that coma and events in the Battle City tournament, where four people fell unconscious after their duels (Mai, Jonouchi, Rishid and Bakura), and two of them were confirmed by witnesses to be the victims of sorcery (presumably Rishid and Jonouchi). She then promptly dismisses it all as nonsense.
 * During the flashbacks that Jin Himuro had of back when he was a pro had a list of names which were actually taken from the manga of the Season Zero arc. In an easier example, the name of Yusei's new inmate roommate before he was due to fight Akutsu is Shugo Aoyama, the name of the Gary Stu that the First Yu-Gi-Oh! movie revolved around. (You know, the one that was never released outside Japan?) And of course, Tetsu Ushio, the first baddie to taste the sadistic Penalty Game in the manga is now one of the core secondary characters in 5Ds.
 * Several aspects of Usagi and Seiya's meetings in Sailor Stars mirror Usagi and Mamoru's meetings in the first season of the series, culminating in Usagi confusing Seiya dressed in black and red for Tuxedo Mask, who wears a black and red outfit. (Seiya went as far as to throw a red rose to distract the enemy.)
 * Throughout the Fullmetal Alchemist are scenes where you can recognize certain people traveling about -- like the Curtis', Hohenheim, or a minor one, the mustache man in the white suit.
 * In Rush Valley, you also see automail stalls with photographs of their previous customers, like the man who hijacked the train in the first volume or the army guy in one of the spin-off games.
 * After Father Cornello's assistant outlives his usefulness, Lust lets Gluttony eat him. Later in the manga/second anime end up inside of his stomach and you can see a skeleton wearing some familiar garments.
 * The second-to-last episode of Best Student Council has basically every character that appeared in a single episode show up to act as a diversion.
 * Negima chapter 268 is a flashback full of these; (links are spoilers, btw) there's the ermine detention compound, an appearance of Tosaka, Mama, and their friends, and Nagi saving a girl who appears to be a young Paio II. (NSFW)
 * Pokémon has a Continuity Nod in "A Lean Mean Team Rocket Machine", by having Jessie, James and Meowth use their original Team Rocket motto.
 * Misty's Staryu makes its first appearance in "Clefairy and the Moonstone" and she calls it back before Ash catches up to her. In the next episode when she uses Staryu against him in a battle he checks it out on the Pokedex since he hasn't seen it yet.
 * Happens a lot in Monster. One notable instance is when Eva's flashbacks finally reveal why Tenma got back from his date in episode 4 so quickly.
 * A big one for Gundam, one spanning most of the Universal Century: Syam Vist of Unicorn (set in U.C. 0096) is 112 years old, due to the use of cryonics. Vist is voiced by Ichiro Nagai, the voice of the narrator of Mobile Suit Gundam (set in U.C. 0079).
 * there is also a much more subtle one in Mobile Suit Gundam F91 and Crossbone Gundam through the character Zabine Chareux (the first four letters of his first name and given name spell ZABI and CHAR respectively)
 * In Mobile Suit Gundam: The 08th MS Team, Shiro has a Flashback Nightmare of his colony being gassed by Zeon troops. Secondary official works have confirmed that this was the gassing Cima Garahau was involved in.
 * All the Macross series have some sort of reference to another one of the series. Even Macross Zero, which occurred prior to the original series, has some connection by featuring Roy Fokker.
 * There's an episode in Macross 7 where two songs of Sharon Apple are clearly audible in the background. Even more noticeable since those techno songs sharply clash with the series' resident musical theme.
 * Macross Frontier in particular is chock full of continuity nods: from Ranka singing songs of the previous installments to SMS formations being named after Fire Bomber songs.
 * There is a literal walking Continuity Nod that appears in SDF Macross, Macross 7 AND Macross Frontier: the König Monster Destroid.
 * In Digimon Adventure, Tai uses a set of cards to open a gate to earth while leaving out Agumon's card. In the followup series Adventure 02, Mr. Owikawa uses the same cards but leaves out the Gomamon card while including the Agumon card, sending him somewhere else entirely.
 * All of Misaki's various opponents in Kidou Tenshi Angelic Layer show up in the audience for the fight in the Grand Finale.
 * Quite often do characters in Pokémon Special make references to events that happened in the past, even if said events happened in another region. For example, when Byron learns that Platinum collected six badges in twenty days, he mentions that he heard of a trainer in Hoenn who collected eight badges in eighty days (ie, Sapphire).
 * At the start of the Platinum arc, when Platina meets up with Looker, one of the first things she asks is if he's supposed to get a free bike ride around Sandgem Park and if he's really a comedian, referencing the mix-up that occurred at the start of the Diamond and Pearl arc.
 * The fourth season of the Slayers anime aired in 2008; the ending sequence featured the plane chart noting the hierarchy of the five Mazoku lords, the four Dark Lords, and the Lord of Nightmares suspended in the background. The two spots on the chart reserved for the Mazoku Lords Fibrizo and Gaav are destroyed, nodding to the fact that they were both Killed Off for Real in the second season of the anime...which aired back in 1996...
 * Oddly, in a possible case of potential discontinuity, the spot on the chart for Dark Lord Dugradigdu remains intact, despite having been destroyed in the third season (1997). It is considered Hajime Kanzaka's way of protesting the season, as it is the beginning of divergence from his books. Since then, he seems to have accepted the change.
 * In chapter 20 of Magical Girl Lyrical Nanoha ViVid, Vivio and her friends are given special bracelets designed by the TSAB Research Department that drains a person's physical and magical powers to help in their training. When Nanoha sees it, she makes a comment about how she used to do something like that when she was a kid, except she had to let Raising Heart manually do the draining for her since she didn't have something as convenient as the bracelets. This was a reference to Nanoha's Training From Hell regimen that was described way back in the first chapter of the Magical Girl Lyrical Nanoha As supplementary manga.
 * In Super Robot Wars OG: The Inspector, during the fight between Axel Almer and Einst Kyousuke Nanbu/Beowulf in the prologue, Axel fire the Soulgain's fist into Beowulf's mech, but it doesn't finish the job. In the final episode, where
 * In the background of the earlier Haruhi episodes, nods to the original series could be seen in the background. Multiple events that weren't shown in the first season, but occurred in the novels, had their remainders shown in the club room. While most were later filled in on in their second season, unfamiliar fans may wonder why there is a frog costume or a bamboo plant lying around.
 * Daily Lives of High School Boys has a constant, but more subtle example in the manga: covers from a given chapter might portray the result or any aftermath from a event that happened in the previous chapter. This is often reproduced as the Eyecatch after the skit corresponding to the said "previous chapter" in the anime adaptation.
 * In another example, in High School Boys and Seniority, Motoharu's sister's classmates came to their house for a sleepover and, among others, were quite interested to have Motoharu shaved. The Eyecatch right after this skit was the manga eyecatch mentioned above, having the girls trying to forcibly shave Motoharu. The next skit High School Boys and the Saviour, we see Motoharu shaved and had a few Band-aids on his chin, clearly the result of being forcibly shaved by the girls.
 * Although Persona 4: The Animation is a much abbreviated adaptation, attentive viewers can note many specific bits from the games that are still present:
 * Though he no longer draws attention to or explains it, Igor repeats the Tarot reading he did in the beginning of the game during the very first
 * Video Game Interface Elements are featured during transitions, fight scenes and the mid-episode Eye Catches.
 * Various Social Link characters are present in the background of scenes they have no involvement in.
 * Yu has a habit of folding multiple origami cranes in his free time.
 * Many scenes that were changed or added to the anime still borrow directly from other events featured or mentioned during the game.
 * A flashback detailing Chie and Yukiko's first meeting in episode 3.
 * Yukiko's pet bird in episode 4, hinted at during the opening cinematic of the game.
 * Adachi coming back from shopping at Junes with nothing but cabbage in episode 7
 * Rise's decision to quit being an idol in episode 9.
 * Personas and Shadows demonstrate abilities related to ones they have in the game.
 * Chie's shadow has her Persona Tomoe's weaknesses in episode 3.
 * Ara Mitama and Pyro Jack are strong against physical and fire attacks respectively in episode 4
 * Shadow Kanji's flunkies have the same immunities as the did in the game in episode 7. Also, Yamata-no-Orochi is fused with the same Arcana combination that would be used in the game. (Magician x Priestess = Moon)
 * During the battle with, when she turns Yosuke, Yu and Teddie into old people, she's actually using her Galgalim Eyes attack, which causes the Enervation status. The connection to the game is further established by having Kintoki-Douji use Energy Shower to reverse the effect - in the game, the skill Energy Shower cures the Enervation status effect.
 * also fights like she does in the game, by exploiting weaknesses. Yu keeps having to change Personas because she keeps preparing to use attacks that his current Persona is weak to (though thanks to Rise, Yu is one step ahead).
 * During the Shadow Naoto fight, Yu has Mot use Mudoon, which doesn't work. Naoto's Persona, Sukuna-Hikona, resists Dark attacks.

Comics
"Peter: Good afternoon, Maude. Looking beautiful as always. Maude: Oh Peter, you lie beautifully darling, have ever since the ninth grade, so don't stop. "
 * Too many to count in The Sandman, but the most memorable is the "Endless Nights" story, "The Heart of A Star" -- Superman's ultimate origin is revealed to be a conversation between Krypton's sun and the first personification of Despair on the nature of beauty.
 * Don't expect this to ever be considered official DC Universe continuity however, as it basically says that Superman only survived so he could feel the despair of being the last Kryptonian.
 * This story also features a character called Killalla of the Glow, who is implied to be one of the first aliens to master the energy later marshaled by the Green Lantern Corps.
 * Heck, she's as-good-as-stated to be one of the Guardians, before the female Oans went their separate way.
 * It's never directly stated, but there are enough hints to figure out that Matthew the raven was, from Alan Moore's series , before he died. Gaiman owes a lot to Moore in getting his career started; there are several nods in Sandman to Moore's work in the DCU.
 * A throwaway line in Paul Cornell's Captain Britain refers to Merlin as one of the Higher Evolutionaries, acknowledging the Doctor Who Magazine comic strip's presence in the Marvel Universe for the first time since Alan Moore was writing both characters.
 * in one issue of Excalibur Kitty and Rachel are being accosted by Daleks while queuing at a bureaucraticc office
 * Plus, Merlin's appearance in Cornell's series looks to be a Call Back to the way he looked in the DWM strip.
 * Writers and artists often use scenes set in the Batcave's trophy room to give a Continuity Nod to previous Batman stories. Traditionally included are the giant penny from a 1947 fight with The Penny Plunderer, a T-Rex from a 1946 adventure on Dinosaur Island, and a memorial to Jason Todd, first seen in The Dark Knight Returns.
 * Also, Batman once investigated murders at a movie set, where the movie being shot was a WWI flying action/drama. The misunderstood Villain Protagonist of said film was Hans von Hammer, in our world the main character of DC Comics Enemy Ace series.
 * Elf Quest examples:
 * In the science-fiction "sequel" The Rebels, Gestrelle Luricahn owning elfin artifacts is just part of her job/field of research. The fact that nearly all of those that we see decorating her room had some importance in previous Elf Quest stories is a Continuity Nod.
 * Sometimes panels from earlier comics get "paraphrased", using the same composition and (if possible) characters; for example one in Kings of the Broken Wheel 3 is modeled after one in the very first Elf Quest story ever.
 * In a montage of the Wolfriders' dens in Wild Hunt, Tyleet's can be identified by the ornament that her long-dead adoptive son used to wear as a headband.
 * Quite a few in The League of Extraordinary Gentlemen, which is to be expected as its continuity arguably includes every book ever written.
 * Asterix does this a few times. In Asterix in Britain, you can see a few souvenirs referring to previous adventures on a shelf in Asterix and Obelix's house (including a model sphinx from Asterix and Cleopatra and a Visigoth helmet from Asterix and the Goths).
 * Occurs regularly in Astro City, as befitting a series with a single writer. Most Continuity Nods appear as sidelong references to other characters and events in the chronological past/present, even if the subject hasn't had a published appearance yet.
 * Samaritan briefly mutters "3.2" when he arrives to visit Steeljack in "The Tarnished Angel." This is a reference to Samaritan's Day in the Life story, "In Dreams," where it's shown he keeps track of how many seconds he spends flying from one scene to another.
 * In "Show 'Em All," as Jack-In-The-Box dodges The Junkman's aerosol bombs, he casually mentions having "recent experience" in dodging mid-air explosions. This refers to a single panel from the earlier "Confession" story arc, where Jack-In-The-Box eludes capture from a missile-firing helicopter.
 * Also from "Confession", Brian begins his super-hero career by working as a busboy in Bruiser's Bar & Grill, run by retired Golden Age hero The Black Badge. Both the Black Badge and Bruisers' play small but pivotal roles in the later "Dark Ages" story arc.
 * Kurt Busiek loves to put Continuity Nods in all of his works, helped by his encyclopedic knowledge of comic book history.
 * A subtle one from The Amazing Spider-Man vol. 2 #55: Peter Parker, currently working as a teacher at his old high school, talking to the school secretary (or administrator?) who remembers him from when he was a student:


 * In The Beano Annual 2009 the Ratz (a group of rats from a 2000s Beano comic strip) briefly meet the Nibblers (a group of mice from a 1970s/1980s Beano comic strip).

Fan Works

 * Kyon: Big Damn Hero is littered with them, from mentioning who received their tanabata wishes first to Koizumi giving an explanation to Haruhi over a cup of coffee, to a reference to the source material's Drama CD, where Haruhi wrote a literal Ear Worm and Hare Hare Yukai.
 * And now,
 * Company 0051, a Fan Web Comic set in the Haloverse, includes several:
 * When finding the crashed Covenant ship, John-117 identifies it as a Seraph-class starfighter.
 * John directly cites as having directly violated Article II, subsection 7 of the Cole Protocol, which directly forbids the retrieval of Covenant artifacts and technology until the available party is certain that the item is cannot be tracked.
 * The armor the kids get at the end looks very similar to the Marine BDU from Combat Evolved.
 * The Robotech fanfics Scoop and Valkyrie Nights contain scenes from the canon material, and the dialogue from those scenes were transcribed word-for-word.
 * The Forever Knight virtual continuation FK4 has a bunch of references to small details in the show which came up once and were never mentioned again. For example, Natalie's cat and Tracy's knowledge of tattoos.
 * Stars Above is loaded with references to both franchises that form up its Intercontinuity Crossover, as the authors are insanely rabid fans of both series.
 * The Tainted Grimoire has one where the Camoan native who cheered Luso and Cheney up in Chapter 63 turned out to be the father of the girl whom Luso, Cheney and Hurdy helped all the way back in Chapter 8.

Film - Animated

 * At the very beginning of Madagascar: Escape 2 Africa when the animals leave Madagascar by plane the ship that the penguins stole in the first movie can be seen parked on the beach.
 * Most Pixar films use a variation of this: in most of their films, if you look very closely at the backgrounds in some scenes, there will inevitably be an element from another Pixar film hidden somewhere-not just with their past films, but also their future ones as well.

Film - Live Action
"Elsa: What's this? Indy: Ark of the Covenant. Elsa: Are you sure? Indy: Pretty sure."
 * From the Batman franchise: In Batman and Robin, a scene of Arkham Asylum briefly shows the outfits of the Riddler and Two-Face from Batman Forever.
 * In the second Batman film of the 90s, Batman Returns, there's a line of dialogue that refers to Vicky Vale, the Love Interest from the first film.
 * In the first Alien vs. Predator film, the expedition to Antarctica was financed by the Weyland Corporation and includes Charles Bishop Weyland himself. In Requiem, the Predator's plasma gun is turned over to the obviously very-well connected Ms. Yutani. As every fan of the Alien series knows, Weyland-Yutani was the major corporation in the background of the films, at the time of Aliens 3 led by the obviously many time great Identical Grandson of Charles Weyland and the model for the Bishop androids (with Lance Henrikson playing all three roles).
 * Every Pirates of the Caribbean movie to date has included someone's remarkable survival and escape from an isolated island being explained as "Sea turtles, mate!" Except that in two of those cases we actually know what happened, while the dog's survival into the third movie is never explained.
 * Another one comes in the second movie, when Jack Sparrow says that Will Turner has a "lovely singing voice" because he's a eunuch (he's not). This is a nod to the first movie, which had multiple jokes about Will being a eunuch.
 * Indiana Jones
 * Indiana Jones and the Last Crusade. Indy casually identifies a tomb painting of the Ark of the Covenant, as recovered by Indy in Raiders of the Lost Ark -- complete with appropriate musical motif from that movie.


 * Indiana Jones and the Kingdom of the Crystal Skull is full of them. From ancient artifacts to family photos there are dozens of references to the previous movies and to when they Recycled: the Series.
 * A similar nod happens in Kingdom of the Crystal Skull, where you see the actual Ark, which has been packed up and stored in Area 51.
 * The James Bond film Die Another Day contains Shout Outs and Continuity Nods to all of the previous Bond films. Q's lab, for example, contains gadgets from a number of previous films, such as the jet pack from Thunderball and the crocodile sub from Octopussy.
 * In addition, when Q gives Bond his new watch, he says, "This will be your twentieth, I believe." Die Another Day was the 20th Bond film.
 * It also references several things in the Bond books that didn't make it into movies, such as the scene with M talking to Bond through bulletproof glass.
 * In On Her Majesty's Secret Service James Bond resigns and is cleaning out his desk. He takes out Honey Ryder's knife and belt from Dr. No, Red Grant's garrote wrist watch from From Russia with Love and the aqua breather he used in Thunderball. All this while as music from each of the previous movies are played for each item. Perhaps this was a way of establishing that the new 007, George Lazenby was the same Bond of the previous adventures where he was played by Sean Connery.
 * In a rare continuity nod for an action film, Jay in Men in Black 2 actually explains where his former partner El went before he hooks back up with Kay--apparently, she liked being a morgue worker more than an MIB. We also learn that Kay's wife left him, which not only gets her out of the way for Kay to go back to the MIB, but is also a refreshing change from the usual fare of just ignoring that the previous love interest ever existed.
 * Serenity has an extremely subtle continuity nod during Mal's Shirtless Scene, where one can see a small vertical scar on his upper right chest. This is the same scar he picked up in "The Train Job" when Crow threw a knife at him and hit him in that spot.
 * There's also a scar in the center of his chest from the torture device in "War Stories".
 * In Speed 2, a cruise liner crashes into and demolishes an expensive boat belonging to the same guy whose sports car Jack (Keanu Reeves) wrecks in pursuit of the bus in the first movie. And Alex commandeers one of the guy's smaller boats to save Annie.
 * Similar to the Batman example above, the 2007 Teenage Mutant Ninja Turtles movie had a scene near the end where Splinter had set up a trophy room. Shredder's helmet and various other artifacts from enemies of the previous three live-action films could be seen on shelves.
 * Mark from The Gamers shows up a couple of times in the second movie and makes references to it.
 * Kevin Smith's movies, particularly those set in The View Askewniverse, are full of Continuity Nods, with characters frequently making reference to events, characters, or relatives of characters that have either happened or been alluded to (or will be) in other movies.
 * The 2009 Star Trek movie features a brief reference to Admiral Archer, who keeps beagles, an obvious reference to the star of Star Trek Enterprise (and his owner, Captain Archer).
 * Also, one of the members of Kirk's disciplinary board is an Admiral Komack - which was the name of of an Admiral in TOS.
 * A lot of nods in Star Trek: during the Kobayashi Maru test, Kirk is eating an apple nonchalantly. In The Wrath of Khan, Kirk tells Saavik how he beat the test--while eating an apple.
 * Though according to the DVD commentary on the 2009 film, this parallel was unintentional.
 * There's a blink-your-ears-and-you'll-miss-it call for Nurse Chapel at one point.
 * Scotty had a pet tribble, in his Reassigned to Antarctica base.
 * For the most part, George Lucas ignores the Star Wars Expanded Universe. But Coruscant, the capital of the Old Republic, was established in The Thrawn Trilogy. The prequel trilogy continues to use Coruscant.
 * And it was retconned into the ending of episode VI, when all planets celebrating are shown (Naboo is retconned into this too, if you must know. As is Hayden Christensen as the ghost of Anakin.
 * Actually, this scene was changed twice: first for the "Special Edition" theatrical re-release, and again for the trilogy DVD release.
 * In the Special Edition of A New Hope, Dash Rendar's Outrider ship from Shadows of the Empire can be seen landing in Mos Eisley.
 * In Spider-Man 2, the staff of the Daily Bugle is trying to think of a name for the recently altered Dr. Octavius. One suggestion is Dr. Strange which is rejected as already taken, presumably by Marvel's Sorcerer Supreme.
 * The Lord of the Rings films feature several nods to other books by Tolkien (though sometimes only in the EE): the allusions are much more explicit in the books, but in the films they can be easy to miss or obscure. See for instance the trolls (from The Hobbit) which are glimpsed in FotR and Aragorn singing the Lay of Leithian (from The Silmarillion).
 * The terrorist group from the movie Iron Man is called The Ten Rings. The Mandarin, a major Iron Man villain from the comics, gets his powers from ten magical rings (though later it's retconned to be Shi'ar technology)
 * Raza, the leader of the Ten Rings, even mention "the man whose ring I wear", i.e. The Mandarin.
 * In Rocky II Rocky takes Paulie's old job at the meat packing plant. During a montage of him doing grueling, manual work there Rocky playfully hits a hanging side of beef like he did to train for his fight with Apollo in the first movie.
 * Rocky Balboa is full of these. Almost every (surviving) character who appeared in the original film (Little Marie, Duke, Spider Rico, Andy the bartender) returns. In a deleted scene (cut because of disagreements with Carl Weathers), Rocky has a photo of Apollo Creed in his house. It's mentioned in dialogue that Rocky's statue (which was unveiled in Rocky III and seen in Rocky V) was removed from its place at the Philidelphia Museum of Arts. The film opens with the song "Take You Back", which was performed in the original film by Frank Stallone. Finally, the training sequence once again has Rocky run through a Philadelphia park and up the Philadelphia Museum of Arts set of steps.
 * Airplane 2 The Sequel. While talking to Buck Murdock on the radio, Ted says "Roger, Murdock". Roger Murdock was the character in Airplane! played by Kareem Abdul-Jabar.
 * The third Mission Impossible film makes use of "The Plot," a music cue by Lalo Schifrin from the original series that is only familiar to fans.
 * The movie Predators had the female lead made a mention of the first Predator and about the story of how Arnold Schwarzenegger's character, Dutch, managed to defeat a Predator after it slaughtered his entire team.
 * In the original Halloween, Laurie mentions having a crush on one of her classmates Ben Tramer. In the sequel, the police accidentally kill a costumed teenager after mistaking him for Michael Myers. Turns out that poor teenager was Ben Tramer.
 * Quentin Tarantino's films are full of in-universe references. Characters from distinct films can be members of the same family (most famously the Vega brothers, but Donny "The Bear Jew" Donowitz from Inglorious Basterds is the father of film producer Lee Donowitz from True Romance). A particularly obscure example is found with Jules' from Pulp Fiction, who quotes the same bible verse before killing people as did the Hatori Hanzo in the Japanese TV and film series, Shadow Warriors(Kage no Gundan), which starred Sonny Chiba. Hatori Hanzo, once more played by Chiba, is the sword smith who appears in the Kill Bill films, bringing the reference back around. Indeed, the quote Jules uses is actually a retranslation from the Japanese scripts and not the version as it appears in English language editions of the bible. It can be noted that in Shadow Warriors, Chiba plays members of the same family from different generations, most of whom share the same name, with the Kill Bill character seeming to be a further descendant.
 * Several throughout the RoboCop series:
 * In Robocop 2, the titular character brings a Cobra Assault Cannon (the weapon used by Boddicker and his gang in the first film) to the final showdown with Cain, but it is swiftly rendered useless after the bullets fail to impede the killer cyborg.
 * In Robocop 3, the "I'd buy that for a dollar!" television host is seen in a brief cameo, while an OCP officer named Cecil (who attempted to stop the officers from destroying Robo in the OCP parking garage during the first film) returns as an officer who walks out on OCP and helps Sergeant Reed during the Splatterpunk attack in Old Detroit.
 * In Scream 4, a girl in the beginning gets crushed by a garage door, harkening back to a similar death from the first film.
 * In The Godfather, Enzo Aguello (a baker from Sicily) helps Michael Corleone when assassins attempt to kill Don Corleone at the hospital he's staying in, by standing guard outside the building and waiting for the police to arrive. He returns in Part III as the man who bakes a cake for Michael when he receives the Order of Saint Sebastian.
 * Two very different films are established as being in the same 'verse by one of these. In John Landis's 1978's Animal House, the "what happened to...?" epilogue states that Neidermeyer was fragged by his own troops in Vietnam. In the segment of 1983's The Twilight Zone movie that Landis directed, the focus is on a particular group of soldiers, one of whom makes an offhand comment about having "fragged Neidermeyer".
 * The Fairy Tail manga is incredibly good at this, to the point that it's actually too much of an effort to list all the small background details in it, though special mention goes to the Continuity Porn that is Natsu's house and the manga's willingness to refrence events from filler episodes of the anime.

Literature
""No, sir, of course not. Beren now, he never thought he was going to get that Silmaril from the Iron Crown in Thangorodrim, and yet he did, and that was a worse place and a blacker danger than ours. But that's a long tale, of course, and goes on past the happiness and into grief and beyond it - and the Silmaril went on and came to Eärendil. And why, sir, I never thought of that before! We've got - you've got some of the light of it in that star-glass that the Lady gave you! Why, to think of it, we're in the same tale still! It's going on. Don't the great tales never end?" (The Two Towers, "The Stairs of Cirith Ungol")"
 * Peter David's Star Trek: New Frontier novels (part of the Star Trek Expanded Universe) include an immortal character named Morgan Primus. It's hinted that the character may be the same as Number One (from "The Cage"), suggested that she is the original model for shipboard computers' voices, and both Jean-Luc Picard and Montgomery Scott mistake her for other women they're familiar with (Lwaxana Troi and Christine Chapel, respectively). All of which is a big wink at the numerous roles that actress Majel Barrett (-Roddenberry) has filled in the Trek Verse.
 * David loves these. In Q-Squared, the bartender in one of the parallel universes is not Guinan, but Yeoman Caryn Johnson.
 * The popular "it's a trap!" quote and meme from the Star Wars films is given a Continuity Nod twice in the Legacy of the Force novel series; once by Mara Jade in Sacrifice, and once by Luke in Revelation.
 * In Dark Force Rising, Luke and Mara sneak aboard a Star Destroyer to rescue Talon Karrde. Luke sneaks into and out of the detention area via a trash compactor. (Also, earlier, looking to disguise themselves for the rescue attempt, Luke advises Mara that stormtrooper armor is hard to see out of.)
 * "I can't see a thing in this helmet!"
 * References to "I have a bad feeling about this" are rife in the Star Wars Expanded Universe. And Death Star is absolutely packed with acknowledgments about the builders of the Death Star, the prototypes, and what else is going on in the galaxy.
 * Star Wars Legacy had an "I love you." ... "I know." exchange between two characters, one of whom is a descendant of Han and Leia.
 * The Star Wars EU is unusually continuous, for an Expanded Universe. In theory all books are a view of the same 'verse; some views are crystal clear, some are blurry, and some are downright abstract. In practice it depends on the writer. Karen Traviss does no research at all and merrily tramples on previous canon, then calls people who dislike this the "Talifan". Timothy Zahn reads everything, takes it into account, and then incorporates details and characters freely and subtly, though he retcons things here and there. In the first book of the Hand of Thrawn duology, he introduced the Caamaasi species, though none of them were named characters. Immediately after that, Stackpole wrote "I, Jedi" which had a Caamaasi character, and in the second book of the duology Zahn included this character in a fairly prominent role. The two books of the duology were published a year apart. The character was in character. Most writers lean more towards Zahn than towards Traviss, but is it any wonder who is more loved?
 * While authors try to mimic Zahn in this way, Lucas Arts itself seems to merely despise the fanbase when they ignore the idea that there is any pre-established continuity. They ultimately decide the outline of new Star Wars material, and in an actual inversion of the Continuity Nod that goes beyond simple Retcon, they add new events to the timeline thousands of years apart and then connect them together. For example, the new Fate of the Jedi series, which takes place about 40 years after the movies, involves plot elements from 5,000 years before movies. Previously, the span of important events covered to about 4,000 years before. When they ran out of room to reference events, they just tacked it on the end, and at an arbitrary date no less. Plans spawned from said 5,000 year plot must coexist alongside other plans, such as Palpatine's rise to power, The Sith Empire from The Old Republic hiding in the Unknown Regions, and The One Sith (which is itself another example of this): all of these plans manipulate politics and events at a galactic level, yet they never contradict one another. This new element is simply the newest of a long trend, and one that just ramps it Up to Eleven. The characters are unaware of these plots (as is the audience) but the Galaxy Far, Far Away, in its omnicience, knew about them the whole time.
 * Little windows to the first book of Harry Potter abound in the seventh, and show starkly just how bad things have gotten.
 * From the chapter "The Prince's Tale": "Keep an eye on Quirrell, won't you?"
 * There are about a million of 'em in that chapter as Harry goes through all of Snape's relevant memories--Dumbledore and Snape's conversation in the woods (where Snape says "you take too much for granted") is the one Hagrid overheard, as reported in HBP, we hear "that awful boy" (Snape, not James) telling Lily about Dementors, as overheard by Petunia and reported in the beginning of OotP, etc, etc...
 * Book Five is basically one Continuity Nod after another. When Harry takes his exams, he reminisces over all the stuff he did and learned over the past five years.
 * Warhammer 40000: Gaunt's Ghosts has quite a few nods to other books Dan Abnett has written, and almost all of his books include references to the events and characters in his other works. Many, however, are retroactive, having been written about first as a passing reference, then developed into a full story. This can result in circular Continuity Nods.
 * Dan Abnett's Horus Rising opens by recounting how Loken would say that he was there when Horus killed the emperor. This is, in fact, the emperor of a planet they conquered while he was still a loyal son of the Emperor, but it's not lacking in Irony.
 * Indeed, if you know the backstories well enough, the entire Horus Heresy series contains at least one example per chapter that is just like that. Fans will often yell 'No! No! Don't do that! You stupid plonker!' at certain actions, and frankly groan at the various lines that are just dragging the currently on-top-of-the-galaxy Horus
 * Also, note that the first chronological literary appearance of the Dies Irae was in the very first Heresy novel, and it went on to play a major part in the series. Dies Irae appears ten thousand years later as a Chaos Titan that played a major part in Graham McNeill's Storm of Iron novel, written long before the Heresy novels.
 * One of the first Ciaphas Cain books makes reference to there being record of a Commissar who held the dual rank of Colonel-Commissar, a Shout-Out to Abnett's Ibram Gaunt.
 * Discworld has quite a few. One of the bigger ones is a scene in Making Money where Moist is told "You can walk out of that door over there, and the matter will not be raised again." As the last time this happened, the door opened onto a very deep pit, he is a little wary.
 * In Hogfather, Ridcully brings up the events of Reaper Man: "Remember when we had all that extra life force around? A man couldn't call his trousers his own!" Other books have also made reference to "the Holy Wood incident" and "that music with rocks in debacle".
 * This crosses through different protagonists' books. In Thud! Vimes is complaining about "that pea-brained idiot at the post office" (Stanley Howler, Head of Stamps as of the end of Going Postal), whose cabbage-scented stamps apparently ran into a few unforeseen problems, and in Making Money Moist briefly apologizes to Vetinari about the cabbage stamp debacle when trying to think of reasons Vetinari might have wanted to meet with him.
 * In Night Watch, the entire plot is set in motion by lightning striking a particular house as Vimes falls the University library roof straight into L-Space. This doesn't seem especially meaningful, unless you've read Thief of Time, and know that lighting bolt
 * A scene in Thief of Time nods 'back' in reply, as characters from TOT notice there's a fight going on in Sator Square that is the aftermath of the action at the very start of Night Watch. Which makes sense as we're observing it about 5 seconds later.
 * Also, in The Truth Vetinari asks William if Dibbler had had any managerial position of the press. This makes sense when you realize that Dibbler has almost destroyed Ankh-Morpork several times this way: In Reaper Man he had the snow globes, in Moving Pictures there was the "clicks", then in Soul Music there was Music with Rocks In...
 * Unseen Academicals mentions that the last time the wizards got into a succession war, The End of the World as We Know It was only narrowly averted by Rincewind, wielding a half-brick in a sock. There's tons to later books as well, to the point of Continuity Porn for attentive fans.
 * The Dark Morris Dance from Reaper Man later appeared in Wintersmith as a key plot point. Also, Wintersmith ends with Rob Anybody of the Nac Mac Feegles struggling to read "Where's My Cow", Sam Vimes Jr's favorite book from Thud.
 * Nick Hornby makes nods towards his other novels:
 * In About a Boy, Will shops in the record shop from High Fidelity.
 * In How to Be Good, one character lives in a flat in the same building as an employee of the same record shop.
 * In A Long Way Down, a character from How to Be Good has a show on Martin Sharp's terrible cable TV channel.
 * The Doctor Who New Series Adventures novel Beautiful Chaos is a sequel to the TV story Masque of Mandragora, spinning from the Doctor's claim that the Helix would return after 500 years. It takes a moment to refer to the three other Expanded Universe media which took up this plot thread (a Past Doctor Adventures novel, a Sarah Jane audio drama, and a Doctor Who Magazine comic strip), explaining that they all involved "fragments" of the Helix, but this was the real thing.
 * All sorts of nods to past continuity (notably the Third Doctor era) were referenced in the Virgin publishing novel "Who Killed Kennedy", where the plotline worked in many of the Doctor's adventures, from a ground level point of view. In his authors note, David Bishop acknowledged the Kurt Busiek/Alex Ross graphic novel "Marvels", which gave a ground level view of the Marvel Universe.
 * Many of the Doctor Who Expanded Universe works feature continuity nods, usually to the series, but often to other EU works by the same author. One subtle one: in the Eighth Doctor Adventures novel Seeing I (set on an Earth colony in the 23rd century) the Doctor is in a prison/mental institution called the Oliver Bainbridge Functional Stabilisation Centre, where people who Know Too Much are held. At the end of the Past Doctor Adventures novel Blue Box by Orman (set in 1980s America), we're told that the villain ended up in the Bainbridge Hospital, which appears to be the same thing, only run by the US Government instead of a Mega Corp.
 * In the Sherlock Holmes stories, Dr. Watson was fond of making references to other cases Holmes had worked on. While many of these were to incidents that Conan Doyle never based stories on, a few were references to other stories found in the Sherlockian Canon.
 * In John Mortimer's Rumpole of the Bailey stories, Rumpole (the first person narrator of all but a few of the stories) often makes reference to other cases he's worked on (with Mortimer going so far as to add footnotes referencing the title of the story, the title of the compilation volume in which it appeared, and the page number it started on).
 * The Rumpole series is also noteworthy for perpetrating what may well be the greatest Noodle Incident / Continuity Nod transformation in the history of literature. For years, in just about all of the stories Rumpole would reference his greatest case, "The Penge Bungalow Murders" (which, as he often noted, he tried "alone and without a leader"). Finally, nearing the end of his writing career, Mortimer finally wrote the novel Rumpole and the Penge Bungalow Murders, neatly turning that Noodle Incident into a Continuity Nod.
 * In the novel Cetaganda, from Lois McMaster Bujold's Vorkosigan Saga, Miles was on the titular planet to attend a state funeral, and was talking privately with one of the keepers of the Star Creche, haut-lady Rian. The discussion is interrupted by a call to Rian from a Cetagandan agent, ghem-Colonel Millisor about tracking down some useful genetic line. Millisor was the antagonist of an earlier novel, Ethan of Athos, which took place at the same time but was written ten years earlier. The plots have nothing to do with each other outside of this one call, though Miles does note that this will be a useful fact to distract Illyan with when he gets back to Barrayar and has to report. Presumably this led to the events of Ethan of Athos, which in turn led to the mission Miles gets sent on in the short story "Labyrinth", which introduced several major supporting characters.
 * That story results in him being given, in his words, "a lead weight, suitable for sinking small enemies." . Ivan asks if he'll ever wear it, and he says only if he ever needs to be really obnoxious. In Memory the need arises, and it is briefly commented on.
 * Also, in both Komarr and Diplomatic Immunity reference is made to his mother's infamous "shopping trip."
 * The Lord of the Rings, in addition to many more or less obscure allusions to events from The Silmarillion and The Hobbit, manages to do an odd little in-universe continuity nod as Sam realises that he is indeed part of the same continuity, which are legends to him.

"... "My father, though some of his ventures were undoubtedly illegal, was... is... a noble man. The idea of harming another creature would be repugnant to him." Holly tugged her boot from eight inches of snow. "So what happened to you?" Artemis's breath bloomed in icy clouds over his shoulder. "I...I made a mistake.""
 * Artemis Fowl gets a very nicely hidden one. For those who read second book enough, they'd realize that the 6th book revolves around something very briefly mentioned in the 2nd on page 120.


 * Artemis briefly mentioning the event that the 6th book's plot revolves around.
 * Without foreknowledge, the "mistake" he was referring to could just as easily refer to him abducting Holly.
 * Understandably, this happens in every single Hardy Boys book ever.
 * H. Beam Piper example: in Uller Uprising, there's a romance between General Carlos von Schlichten and his adjutant, Paula Quinton. In Federation, the short story "Oomphel in the Sky" has a reference to a Paula von Schlichten Fellowship.
 * Pelevin's The Sacred Book of the Werewolf contains several references to his earlier "A Werewolf Problem in Central Russia", and
 * Used in-universe in Watership Down, in the legends of El-ahrairah. In the "Black Rabbit of Inle" tale, the folk hero's whiskers are gambled away, then re-grown at the end. Days later, as he's beginning the "Rowsby Woof" story, Dandelion mentions El-ahrairah's new whiskers.
 * Tons and tons of them in the modern novel line from the Star Trek Expanded Universe. Just one example: in Star Trek: A Time to Kill, President Zife ends up getting a list of crises which occurred on his watch recited to him. These are, of course, all references to other novels. There was the Genesis Wave, the Holostrike, the Trill debacle, the Selelvians...
 * In the Doctor Who novel The Last Dodo, the Tenth Doctor names the titular bird "Dorothea", but never explains why. Dorothea "Dodo" Chaplet was a companion of the First Doctor.
 * Used extensively in the Age of Fire Series. The three separated siblings often her tales and legends of other's feats, factions from previous stories reappear and affect others, and some recurring characters are met by all 3 siblings.

Live Action TV
"Davros: "Impossible. That face...after all these years." Sarah Jane: "Davros. It's been quite awhile. Sarah Jane Smith. Remember?" Davros: "Oh, this was meant to be. You were there on Skaro, at the very beginning of my creations!""
 * In The Big Bang Theory
 * Episode The Friendship Algorithm, Howard Wollowitz says he's thinking about growing a mustache and that George Clooney has one now. In The Electric Can Opener Fluctuation from the following season, he grows a giant beard while at the North Pole, then keeps the mustache, calling it "the Clooney."
 * Penny's Check Engine light
 * That one is more of a Running Gag
 * The flashback episode "The Staircase Implementation" had a few nods to previously-referenced plotpoints, such as Joyce Kim, Leonard's short-term girlfriend, and the former resident of Penny's apartment, a transvestite.
 * In "The Friendship Contraction", Howard refuses to take Sheldon to the model train store, a hobby he acquired in "The Pulled Groin Extrapolation".
 * The Batman cookie jar from "The Bat Jar Conjecture" can be seen on the kitchen counter in later episodes.
 * Likewise, the sword Longclaw that Leonard and Sheldon purchase in "The Russian Rocket Reaction" can be seen on their wall beside the door in following episodes.
 * iCarly: Despite the show's episodic plots relying on Status Quo Is God, combined by Nickelodeon airing them Out of Order, there are still a lot of elements within the show's universe that nod to episodes from early in the show's timeline: Spencer's sculptures often stick around on set, including the hammer that almost killed Carly staying lodged in the wall, the iCarly iWeb trophy in the studio, and Freddie's giant pie spycam from way back in Season 1.
 * iThink They Kissed is a very good example. The interrogation scene between Carly and Freddie recalls plots from a lot of previous episodes: iKiss, iPromise Not To Tell, where Carly overpowered Freddie, iSaw Him First which referred to when Freddie's voice got lower, and iTwins, with Freddie still not believing that Melanie really exists.
 * iBeat the Heat guest stars the residents of Bushwell who had appeared in earlier episodes, among whom are Chuck, Griffin, Mr. Klemish, Dr. Dresdin and Lewbert, who even had hilarious dialogues with Mrs. Benson involving their failed relationship.
 * In iPsycho, Spencer tells Gibby that he got kicked out of sleep-away camp, but doesn't say why. In the earlier episode iTwins, he consoles Freddie about the "Clown Day" incident by telling about how the other campers tricked him into thinking it was "Naked Day."
 * Caprica occasionally does this, oddly enough most often through musical notes, such as using the Adama family theme during Tamara's funeral. There's also, of course, the "by your command" line from at the end of the pilot, which is itself also a Mythology Gag for the old show.
 * Several on the new Doctor Who series:
 * In "World War Three", when Harriet Jones asks the Doctor about his Northern accent, Rose answers (as the Doctor answered Rose when she first asked him about it), "Lots of planets have a north."
 * "Dalek": Several of the Doctor's enemies from the old show are on display in an underground museum.
 * "Bad Wolf": The Face of Boe is described as the oldest inhabitant of the Isop galaxy. The Isop galaxy was the setting of the William Hartnell story The Web Planet.
 * In the same episode, the security guards on the Game Station threaten to send the Doctor to the Lunar Penal Colony, a direct reference to the Third Doctor story Frontier in Space, in which the Doctor really does get trapped on the Colony.
 * And both "Bad Wolf" and "Parting of the Ways" had a transporter that left behind what appeared to be ash, echoing "The Twin Dilemma".
 * In "The Satan Pit", the Doctor mentions the inhabitants of Daemos, attributed in "The Daemons" as the source of demonic imagery in human mythology, among the races who share similar Satan-like characters in their mythologies.
 * In "Tooth and Claw", the Tenth Doctor arrives in 19th-century Scotland and uses "Dr. James McCrimmon" as his alias. Jamie McCrimmon was one of the Second Doctor's companions, hailing from 18th-century Scotland.
 * In a hilarious Continuity Nod In Joke, in the Series 4 episode "The Poison Sky", when the Doctor is given a Gas Mask by the UNIT General, the Doctor remarks "Are you my Mummy?".
 * From the same episodes in Series 1, Jack Harkness mentions Pompeii and uses the phrase "volcano day." The Doctor uses the phrase again in "Fires of Pompeii" when he and Donna figure out where they've landed.
 * At the end of "Turn Left", Rose's appearance and the upcoming danger is heralded by the words "Bad Wolf" appearing everywhere the Doctor looks. This is a shout out to the ongoing mystery of the first series, the repeated words Bad Wolf appearing everywhere they went, but not with nearly as much frequency.
 * Obscure one: In Destiny of the Daleks (Tom Baker era, late 70s), quoth Davros: "Doctor, do you believe your puny efforts can change the course of destiny? ... Destiny, Doctor.... Invincible necessity.... Power. My power. My invincibility. My supreme plan to control the universe... Errors of the past will be rectified. I will add new design elements to the Daleks' circuitry. They will be armed with new weaponry. Weaponry so devastating that all matter will succumb to its power! I will equip them with all the knowledge of the universe! ... The Daleks NEED ME!" -- In "The Stolen Earth"/"Journey's End" at the end of season 30 (aka Series 4 in the new ordering), Davros
 * Probably the longest delay before a continuity nod, in "The Stolen Earth" the Doctor mentions, "Someone tried to move the Earth before, but that was a long time ago...", calling back to the Hartnell era in which  (If you wondered, 45 years. 45 years!)
 * From the immediately following episode "Journey's End":

"Martha: Wait a minute--there was an earthquake in Cardiff a couple of years ago. Was that you? Tenth Doctor: Oh, that was a long time ago. I was a completely different man back then."
 * Not as far reaching as most of the above, but in the finale of season 4, the Doctor and Rose ask if Gwen from the Torchwood team has a family history in Cardiff. She says that she does, going "back to the 1800s," indicating that the character Gwynneth from "The Unquiet Dead", played by the same actress, shared "spacial genetic multiplicity" with Gwen's family.
 * "Voyage of the Damned" nods to the previous two Christmas Episodes by having everyone realize that London isn't a good place to be on Christmas unless you like being at ground zero of an alien invasion.
 * So at the conclusion of The End of Time, the Tenth Doctor is about to regenerate into Matt Smith and say his last goodbyes to his companions over the years. And then there's Alonso from "Voyage of the Damned". And THEN there's the great-granddaughter of Nurse Redfern "Human Nature"/"The Family of Blood".
 * His good bye to Alonso is both nice and a bit thoughtless. He hooks Captain Jack Harkness up with Alonso. However, the scene becomes alot darker if you know why Jack is there and not with Torchwood at the time.
 * In "School Reunion", Rose argues at companion from the 1970s (or the 1980s) Sarah Jane over who faced the toughest aliens and monsters, referencing actual episodes of the classic and new series, including the previous week's.
 * During The Sarah Jane Adventures, when she first encounters the Slitheen, Sarah Jane remembers that argument, saying aloud, "Slitheen in Downing Street". The Slitheen also mention their routine job where the family members never came back.
 * At the beginning of "Utopia," there's a mention of the events of "Boom Town" (the last time the TARDIS had to refuel in Cardiff) and the Ninth Doctor:

"Napoleon Solo: Very ingenious. Am I to assume I'm in THRUSH Headquarters? Oregano: When we reach the 13th floor. Napoleon Solo: Mmm. That haberdashery shop... Oregano: What about it? Napoleon Solo: It's quite impressive. I wish U.N.C.L.E. could afford that."
 * In the next episodes continuing the story of "Utopia,"  offers his wife a jelly baby. A bit of an Ironic Echo as well.   also mentions the Doctor fighting off Sea Devils and Axos, both from Jon Pertwee's tenure in The Sea Devils and The Claws of Axos.
 * The 11th Doctor's fez-and-mop combination mimics Seven in Silver Nemesis.
 * Quite possibly the biggest continuity nods are to the other doctors. The biggest one ever was when The Eleventh Doctor is facing the Atraxi in the Series 5 premier and asks them if the Earth is protected. The Atraxi looks at all the various monsters that show up in the series (including those that don't attack earth), then the Doctor asks "What happened to them?" Cue a twenty second montage of every single Doctor from William Hartnell to David Tennant with Matt Smith walking through the image of Tennant to say "Hello. I'm the Doctor."
 * Buffy the Vampire Slayer:
 * One of Xander's roles on the show (possibly an aspect of his role as the "heart" of the team) was pointing out the similarities of their current situation to plots from the past.
 * In Buffy's Season 2 finale, Willow is going to attempt to restore Angelus's soul. Xander comes to Buffy knowing this, but he chooses to lie, telling Buffy that Willow said to "Kick his ass." Nothing is made of this until five years later, on a Season 7 episode, Buffy refers to the "Kick his ass" message in front of Willow and Xander. Willow indignantly denies the allegation, Xander glosses over the awkward moment, the subject is changed and nothing is said of it again.
 * In the second series episode "Phases", Oz remarks, while looking in the school's trophy cabinet, that the eyes of one of the statues seem to follow you wherever you go. This is a direct reference to series 1 episode 3 where the spirit of a witch was - unbeknownst to the characters - trapped inside said statue.
 * In the Season 6 episode "Hell's Bells," when Willow talks to Xander, both decked out in a dress and tux, respectively, she remarks it's a good thing she realized she was gay because of their actions in formal wear, referencing Season 3's "Homecoming" and the resulting arc.
 * The Season 8 comic-book continuation (outlined and written in part by Joss Whedon) goes as far as to reference events that happened on Angel final season. One of them retconned the fact that Buffy was dating a vampire called the Immortal.  The other is less justified, as it involves Buffy making reference to something she never appeared to know on the series, though she could have learned it by Andrew in the meantime:.
 * In the Season Seven episode "Him", Buffy tries to kill Principal Wood with the rocket launcher she used to kill the Judge back in Season Two.
 * In the same episode, Willow mentions the danger of love spells. Xander has a brief flashback to a season two episode in which all the girls in Sunnydale (save Cordelia) fell in love with him due to a love spell.
 * Don't know if it's intentional or not but Xander wears a particularly "nice shirt" in season one that Oz wears in season three. There's a Slash Fic series that explains the backstory for the transfer.
 * When Willow and Oz meet for the first time, Oz only says one thing: "Canapé?" and offers her a plate of it. When Oz and Bayarmaa meet for the first time, Bayarmaa only asks, "Butter tea?" and offers him a plate of it.
 * Angel, as a Spin-Off of Buffy the Vampire Slayer, occasionally included Continuity Nods to its parent show. One good example was an episode where Angel tracked a girl with Telekinesis who accidentally TKed a steel rebar into his chest. Being a vampire, that didn't kill him, but the pain distracted him enough that he didn't get any information on the girl, not even her name, before she ran off, making helping her difficult. When Cordelia chastises him for it he says "Do you know how hard it is to think straight when you've got a steel rebar embedded in your torso?". She thinks for a second then says "Actually, I do. Benefits of a Sunnydale education". This is a continuity nod, referencing an accident the character had two and a half years ago on a different show, and also a Development Gag: Charisma Carpenter, the actress playing Cordelia, really does know the feeling, she was impaled herself in real-life and the scene was written into Buffy to explain the scar.
 * Angel also has several episodes featuring a woman named "Anne" who works with the homeless. The character actually originated in a season two episode of Buffy as a wannabe vampire named Chanterelle. She shows up again in a third season episode (in which Buffy runs away to Los Angeles and gets a job as a waitress, calling herself by her middle name, "Anne") and is now going by the name Lily. By the end of that episode, "Lily" has adopted the fake name--and job, and apartment--that Buffy has been using, and Buffy returns to Sunnydale. Angel and company are, of course, unaware that Anne is using a fake name taken from Buffy during her brief stint as a runaway.
 * The Cosby Show and its Spin-Off A Different World maintained close continuity ties even after Lisa Bonet left the latter program. Dwayne Wayne once visited the Huxtable home, and a former regular character from The Cosby Show, Charmaine Brown (played by Karen Malina White), showed up on A Different World as a new student at Hillman College.
 * Arrested Development does this very frequently, one of the reasons it is much admired by fans.
 * Third Watch's finale had a photo in the background reporting on events a few years previously, when Ty Davis Jr. helped save lives during a blackout.
 * The HBO series The Wire may be one of the most consistent examples of this trope, going out of its way to reference events and characters from multiple seasons past. Notable examples include: Drug dealer Bodie Broadus referencing a conversation from the beginning of the series when talking to a detective in the fourth season; a former chauffeur named Day-Day being brought in to testify against a senator in the fifth season (after his last appearance was in the first season);, a small boy who was seen trying to emulate his target two seasons prior to the episode; the main criminal organization of the second season (The Greeks) show up again in the final season to do business with drug baron Marlo Stanfield...the list goes on.
 * An addict first appears in Season 3 as a clean-cut college student buying drugs in Hamsterdam. In Season 4, she is seen talking to Old Face Andre, now obviously a streetwalker. And finally in Season 5, she appears in order to give a speech at a Narcotics Anonymous Meeting.
 * And that's not even mentioning the string of characters from seasons past that made brief appearances in the fifth season even if they didn't have major bearing on the season's plot themselves, including but not limited to Nick Sobotka, Clarence Royce, Randy Wagstaff, and Wee-Bey Brice.
 * The Degrassi franchise does this, most notably in its current incarnation, Degrassi the Next Generation. The pilot episode, and many of the scenes involving adults that come later on in the series, are continuity nods. Joey Jeremiah owns a car dealership (he was obsessed with vehicles in Degrassi High), and other characters make cameo appearances (some of which are still affected by events that happened in previous series, like Lucy).
 * Lost has featured several of these, such as allusions to the polar bear in the pilot episode. The episode "Exposé" was essentially one big long Continuity Nod.
 * In the 2008 season opening of NCIS, Tony has been assigned to the (fictional) aircraft carrier Seahawk. The Seahawk was a recurring location on JAG, including being the location of the Pilot Movie. NCIS is, of course, a Spin-Off of JAG.
 * Stargate SG-1 has plenty, ranging from the blatant (anytime Carter's reminded that she blew up a sun) to the very subtle (Jackson always looks a little uncomfortable when someone mentions radiation).
 * After being transformed into a teenager In the episode "Fragile Balance" Col. Jack O'Neill does not convince the others of his identity even after revealing classified information only he could know. Daniel Jackson reluctantly admits stranger things have happened, at which point he is asked to name one. He names several before being interrupted, "well there was that one time we became really old, the time we became cavemen, the time we all swapped bodies..."
 * Other notable nods include: macaroons (in the last two seasons and the films), Vala's first appearance, the time Daniel replied to a question about whether or not he was a Soviet spy in Russian (to be fair, the question was asked in Russian too), the numerous alternate timeline and alternate reality episodes, and "undomesticated equines".
 * The last one is actually referenced repeatedly in-series as being a "callback" to what may have been Teal'c's first joke. It goes so far that it's even mentioned in Atlantis when Sam crosses over.
 * Star Trek Enterprise was practically built around this concept. Considering its status as a prequel, most episodes revolved around the minute facts surrounding the future events like the forming of the Federation. Some range to really excellent uses like the first practical use of the force field and Brent Spiner as a distant Mad Scientist relative of Data's creator, to some groaners like a discussion on the probability of Vulcan/Human hybrids.
 * Turned up on several other Star Trek series' as well. For example, Tom Paris on Star Trek Voyager referencing the Dixon Hill series, Picard's favorite holonovel from Star Trek the Next Generation.
 * In The Next Generation, Worf helps Keiko deliver Molly. When he hears that Keiko is having another baby in DS9, he takes care to be far away when it happens. Very far. "Visiting his adopted parents on Earth"-far.
 * Criminal Minds makes extensive use of this as Character Development -- Garcia's fear of being outside her office, Hotch looking at his phone out of habit, and Garcia and Morgan's relationship being prime examples. The casual viewer would completely miss all this without it affecting the story too much.
 * The Seinfeld episode "The Andrea Doria" features George telling a condo board his entire life story to arouse their sympathy so they'll give him an apartment. Naturally, it consists entirely of references to previous episodes.
 * In the episode "The Puffy Shirt," when George becomes a hand model, he says "I won a contest" to prove that he is Master of his Domain, a reference to "The Contest."
 * Also in reference to "the Contest", in the episode "The Handicap Spot," George can be seen reading a Glamour magazine in the background at his parents' house. In "The Contest," he was caught by his mother doing that with her Glamour.
 * In the finale, when their plane starts falling and they think they're going to die, George admits that he cheated in the contest.
 * In the House episode "Painless", a man walks to the conference room, and asks which one is House. House says: "The big black guy". Later, he tells to Foreman: "Can you blame me? The last time that happened, the guy shot me" -- a reference to the episode "No Reason" (though then, he answered "Skinny brunette" to the same question, and it didn't help him).
 * In "The Itch", the Patient of the Week became agoraphobic after he was mugged. House comments: "Anybody can hate humanity after getting shot. Takes a big man to hate it beforehand." This also can be interpreted as a reference to "No Reason".
 * Used every now and then on How I Met Your Mother. For example with the saluting to things like "private matter", "general idea" and "major clean up".
 * A particular favorite of mine: Barney makes a throwaway reference in one episode to having had sex with Madeleine Albright. In a later episode when we see several of his conquests flash before his eyes, Albright is among them.
 * Another great one was the throwaway reference to Stella's husband, Tony, moving to LA to become a screenwriter. A season later, a movie he wrote based on Ted's life becomes the center of an episode.
 * In an early episode, Barney and Ted's first meeting is shown with Barney stating that they "just met at the urinal". A later episode extends the flashback, showing that the pair did indeed meet at a urinal.
 * In a season 2 episode, Barney reveals that he lost his virginity to a middle-aged neighbor. In season 3, a chance encounter with the woman who plucked his flower results in him losing his mojo.
 * The dating service that couldn't find a match for Ted in season 1 returns with a match later that season, although
 * Law and Order:
 * Jack McCoy goes after a John Yoo-like figure for war crimes. When he's called on it, he mentions a previous episode where he prosecuted a Chilean general for war crimes.
 * In an earlier example, in the episode "Jeopardy," a judge facing charges of taking a bribe - a sweetheart deal on a mortgage on his house in Sand's Point in exchange for favorable rulings in the case of the son of a family friend - commits suicide. Two seasons later, in "Harvest," McCoy makes a reference to a doctor/defendant having a large financial obligation in the form of a mortgage on a house in Sand's Point that he bought from a judge's widow. There's no direct connection made, but alert fans picked up on it right away.
 * Psych does this on multiple occasions, including references to "Chad," the character Shawn played on a telenovela in an early episode.
 * In Mr Monk Gets Cabin Fever, Natalie mentions that people seem to die wherever Monk goes. Stottlemeyer agrees with her, citing a few different Busman's Holiday incidents involving Monk, all of which were previous episodes of the show.
 * In Mr. Monk and the Lady Next Door Monk becomes despaired when he realizes that everyone who quickly becomes a close friend of his is just doing it to get something, recalling how frequently they turn out to the be villain of the episode. Needless to say, this time
 * An interesting case of a literal Continuity Nod on Iron Chef America: in the opening, the Chairman nods in respect to a yellow bell pepper-the symbol of his uncle and predecessor, Chairman Kaga.
 * Used pretty frequently on ER. But then, with 15 seasons, the show has a lot of backstory to refer to.
 * The recent CSI crossover gives us a very nice continuity nod in its first part. When Ray Langston arrives in Miami, Horatio Caine asks him to give his regards to Catherine Willows and gives condolences for the loss of Warrick Brown (both characters traveled to Miami and worked with Horatio in the CSI episode which served as Miami's pilot). Similarly, in the final episode of the crossover, Ray gets texts from both Horatio and Mac Taylor, informing him of arrests related to his case.
 * CSI itself also had smaller continuity nods, recalling events from the first and second seasons in the ninth and tenth - a comatose rape victim and the adopted son of an early recurring foe respectively.
 * In the third season The Man from U.N.C.L.E. episode "The Hula Doll Affair", the entrance to THRUSH Headquarters in New York is an upper-class haberdashery, in contrast to the working-class Del Floria's Tailor Shop which serves as the entrance to U.N.C.L.E. Headquarters. Solo, as he is captured by Oregano (a THRUSH operative), remarks on the contrast:

"Parker: Remember last time I was the carrot? Remember how I stabbed that guy?"
 * Supernatural: Sometime offscreen third season, the Winchester brothers each got a possession-blocking protective tattoo. From that first appearance onward, the tattoos can be seen from time to time over the collar of their shirts, though attention is almost never drawn to them again.
 * Red Dwarf: In "Demons and Angels", whilst testing the food on the 'high' Red Dwarf, Lister refers back to several previous episodes: "I've been to a parallel universe, I've seen time running backwards, I've played pool with planets and I've given birth to twins, but I never thought I'd taste an edible Pot Noodle."
 * Of course, he's also made disparaging references to Pot Noodles before - notably in "Marooned".
 * This is rather a contested issue. In the episode guide, it points out that when Lister did play pool with planets, it was to divert a timeline that had gone wrong and after filling the black hole the timeline adjusted itself so the episode, in effect never happened. It may of course be, however, according to the episode guide at least, that Lister played pool with planets again and lived to remember it - but it wasn't thought worthy of another episode being written to honour this escapade.
 * Bottom: the Sprouts that did not get eaten in Holly make a reappearance in Terror.
 * Scrubs had a huge example in the big Musical Episode. In Carla's song, "For the Last Time, I'm Dominican", she reprimands Turk for mistaking her for Puerto Rican and asks him questions about herself. These questions are all references to previous episodes.
 * "Did I grow up in Illinois or was it Michigan?": In the episode "My Day Off", JD asks Carla something about her "homeland". She tells him she was born in Chicago.
 * "How long before we met was I in medicine?": In the episode "My Nickname", Carla reveals she had been in medicine for nine years before this first season episode. Turk and Carla met in the first episode of the first season.
 * "Was our wedding song the Beatles or Led Zeppelin?": In the episode "My Best Friend's Wedding", Ted and his acapella band, the Worthless Peons, sing the Beatles' "Eight Days a Week" at Carla and Turk's wedding.
 * In the third episode of Thirty Rock, Liz chokes on something she's eating in her apartment and manages to clear her airway by pushing a chair into her gut. In the third season, she starts choking on a date with a doctor so incompetent he doesn't know the Heimlich maneuver, and she does the exact same thing.
 * Castle's first episode of the third season has a callback to the first season (also the opening credits), when Ryan comments on a cardboard cutout of Castle that "He really is ruggedly handsome".
 * Later in the third season (3x10), they find a massive cache of Prohibition era liquor. Nine episodes later, a DA mentions that he just got a bottle of that stash.
 * And then, of course, when things get really hairy, Castle pulls out his custom-made bulletproof vest labeled "Writer".
 * When Marcel the monkey is introduced in Friends, he's sitting on Ross' neck and Chandler remarks "Hey, that monkey's got a Ross on his ass." Several seasons later, when Ross has a strange growth on his posterior region of which even the doctors don't know what it is, Joey suggests that it's something new that might be called for him as in "Poor guy, he's got a Ross."
 * Also in an early episode Chandler mentions he used to have a "Flock of Seagulls" haircut. In a S10 flashback Chandler is seen with a "Flock of Seagulls" haircut
 * Leverage throws these out on occasion. For example, an episode in season one involved Parker getting angry with the mark she was talking to, stabbing him with a fork and jumping out the window. Near the end of season three Nate is walking with her to catch up with the mark on a different con, about which Parker is a little worried.

"Doggett: You ever carry one of these? Scully: ...never."
 * It's also mentioned in Season Four's "The Fifteen Minute Job" when Parker gets excited with her performance after talking with the mark and exclaims "And I didn't even stab him!"
 * Donna and Ringo's wedding in Neighbours is accompanied by Sam Clark's cover of Angry Andereson's "Suddenly". Ringo tells him that Paul put him on to it, having heard it at his brother's wedding years ago. This was, in fact, the song played over Scott and Charlene's wedding in 1987.
 * During the first episode of 3rd Rock from the Sun, the main characters express a fear of Jell-O. When Sally is hospitalized, a nurse comes to visit the crew with Jell-O. Their reaction? Extreme horror.
 * The X-Files has a fair few in its 9 season run...
 * In one episode involving a man killed in a locked room, Scully examines a small ventilation grate, causing someone to ask "You don't think anyone could have squeezed through there, do you?". This is a reference to Eugene Victor Tooms, who killed people in locked rooms by doing just that in an earlier episode. The episode he first appeared in was even called 'Squeeze'.
 * In the episode 'The Field Where I Died', Scully tells Mulder that she wouldn't change a day of their time together - "Except for that Flukeman thing. I could have lived without that just fine." The Flukeman was a well known monster of the week from season two.
 * In 'Dreamland' events occur (which are later forgotten by everyone) in which Mulder's apartment is completely refurnished, acting as a comedic stinger at the end of the show. Several episodes later in 'Monday' his waterbed becomes a plot point, and he's unable to tell anyone why he has a waterbed. "I...I think it was a gift."
 * In 'Patience', one of the first episodes of season 8, where Scully works with new agent John Doggett, they go into a dark attic. Doggett produces a flashlight...

"It's the Mystic Mother! She was known as Rita during the Dark Times."
 * In the second season finale of Modern Family ("The One That Got Away"), Alex and Haley are preparing a video card for Jay with interviews done during the past year. Each of the interviews references a different event from the past season.
 * "Tony's mug" in Twenty Four. In the first season, Tony Almeida is often seen drinking from a mug with a Chicago Cubs logo on the side. It appeared infrequently throughout the first three seasons, and even had a bizarre online fan following. Following Tony's return in season 4, it's revealed that the mug is one of the few things he still has from CTU (used for comedy purposes; he drinks beer from it). The fifth season takes this a step farther and references the mug again when it's smashed after.
 * The "Jack Sack", the Fan Nickname for Jack Bauer's messenger bag he carries in the later seasons. Despite Jack being kidnapped and tortured in the interim between seasons five and six, and moving from Los Angeles to Washington (and then to New York), the messenger bag pops up from time to time throughout the series when Jack is on missions, and conceivably carries anything and everything.
 * When dies at the beginning of Season 5, Jack goes to the crime scene during the next episode, and sees the body lying on the floor of his penthouse suite. The camera focuses on his right hand, which still has the scarring from when he was.
 * Jack's arm tattoo (which he received prior to the events of season 3, when he was undercover in the Salazar cartel) is seen a handful of times throughout the rest of the series, most noticeably in a season-four episode where Jack changes his shirt after an interrogation.
 * In the Community episode "Modern Warfare", Abed's bandoleer of paintball pellets has a slot for his lip balm which is a reference to the earlier Halloween Episode. Also, a seemingly irrelevant scene involving Abed delivering a baby to a nameless character in the background of a random episode leads into him deliver another baby many episodes later. Lampshaded by him, of course.
 * Community is chock full of these. Not only do random secondary characters regularly show up in background scenes, speak a few lines, or get mentioned, but seemingly irrelevant scenes like Abed helping deliver a baby to a nameless character in the background of a random episode helps him deliver another baby many episodes later. Lampshaded by him, of course.
 * In one episode of Home Improvement, Tim soups up a power washer and blasts "Al's a gal" into a wall of the studio building. It's there every time Tool Time does a show outside at that location.
 * In Power Rangers Mystic Force, Mystic Mother is the same character as Rita, in reference to a stock footage character played by, and in tribute to Machiko "Rita Repulsa" Soga:


 * The Waltons: In an episode Jim Bob thinks he's adopted because he doesn't resemble other Waltons physically. Elizabeth conducts a school poll asking students whom they think Jim Bob resembles. One student's answer was, "a turtle". Much later in the series, the Walton boys are dressing for an event, and while Ben and Jim Bob are looking in the mirror, Ben ribs Jim-Bob by snarking, "Elizabeth's right. You do look like a turtle!"
 * Power Rangers in Space had a Badass Long Hair Red Ranger kill Zordon. No doubt this was a previous Red Ranger's greatest fear.
 * Surprisingly for a show that wasn't famed for its continuity, Charmed had a few, particularly in the eighth season.
 * Greg, Piper's one-off boyfriend from season 6 makes a guest appearance when Piper backs her car into him.
 * The Avatars from the seventh season and the Angel of Destiny (played by a different actor) from season four were called on for advice.
 * Billie mentions the sisters' actions in the season 2 episode "Ex Libris" and gets them to use the same tactic again.
 * In "Morality Bites" the spell to go to the future can only be used once. Later on in "Chris Crossed" (four seasons later) Chris says that there is no spell to send someone forward in time (the sisters had already used it so it had disappeared from the Book of Shadows).
 * Later Mash episodes occasionally include mentions of such departed characters as Henry, Trapper, Frank, and Radar.
 * Boy Meets World had a few, including the brief reappearance in season 5 of Shawn's pet pig "Little Corey" from season 3. There were also several references throughout the series to the time Shawn blew up a mailbox in season one.
 * In Torchwood: Miracle Day Jack at one point introduces himself as Dr. Owen Harper. Owen was killed in "End of Days". He was also a doctor before he joined Torchwood.
 * Several characters also reference the 456 Directives, which were adopted after the world-wide incident with the aliens only known as the 456 in the third series.
 * Gwen mentions she wishes the Miracle happened a year ago, so that Ianto wouldn't have been killed by the 456.
 * Done oh so many times on Warehouse 13, usually involving past experiences with artifacts. Some of these reference past episodes, others are Noodle Incidents, such as the one with Abraham Lincoln's hat, causing Pete to have an irresistible urge to free Mrs. Frederic. She was not amused.
 * In The Vampire Diaries Damon hid the moonstone (an enchanted stone which looked a bit like a fancy soap) in the soap dish, where it was promptly found by Katherine the next time she washed her hands, leading to some amusement at his terrible attempt at Hidden in Plain Sight. A season later Caroline is searching for Elena's necklace in the same room, and the show provides a close shot of her digging through the soap dish. This time, all it contains is soap.
 * Another nod to this is when Damon is hiding stakes with his fire wood and Alaric comments on how he should've learnt his lesson from the moonstone in the soap dish.

Music
"King Paramount: I am waiting until a punishment is discovered that will exactly meet the enormity of the case. I am in constant communication with the Mikado of Japan, who is a leading authority on such points."
 * Occurs on a comedy album, of all things. The Lonely Island's Incredibad opens with a song about Carlos Santana's champagne brand, which is referenced in the Memetic Mutation-level I'm on a Boat.
 * And again in the interlude Normal Guy.
 * At the end of Dick in a Box, Andy and Justin Timberlake get sent to jail for indecent exposure. The video for Motherlover opens with them coming out of jail and dropping some boxes in the trash. Towards the end, Justin says, "This is the second-best idea that we've ever had!"
 * During the bridge in Threw it on the Ground, Andy talks about how there are "so many things to throw on the ground". One is a boiled goose, a reference to Boombox.
 * Meat Loaf has the song "Blind as a Bat", with a title blatantly designed to remind of his great hit "Bat Out of Hell", even though the songs have nothing really to do with one another. (One might argue that this applies to the entire Bat Out of Hell "trilogy"...)
 * The Beatles' "Glass Onion" actually approaches Continuity Porn levels, referring directly to "Strawberry Fields Forever", "I Am the Walrus" (with a twist), "Lady Madonna", "The Fool on the Hill", and "Fixing a Hole".
 * "I Am The Walrus" itself contains the line, "See how they fly like Lucy In The Sky."
 * And "Lady Madonna" references "I Am the Walrus" with the line "see how they run."
 * During the fadeout on "All You Need Is Love", Paul starts to sing, "She loves you, yeah, yeah, yeah..."
 * "If I Fell" has a subtler one: "'Cause I've been in love before, and I found that love was more than just holding hands..."
 * The lyrics of the Megadeth song "Victory" consist of previous Megadeth song titles written in a "Previously On..." form.
 * Axel Rudi Pell uses this trope very frequently.
 * "Dark Waves of the Sea" features the exact chorus from "Oceans of Time" as an interlude, along with a similar melody overall.
 * The title track of Mystica refers directly to previous songs and albums by title, such as Kings and Queens, Oceans of Time, and "Edge of the World", among many others.
 * In 1996, R Kelly had a song featuring the Isley Brothers about a man cheating with "Mr. Biggs'" woman. In 2001, the Isley Brothers had a song featuring R Kelly concerning the same issue and the same characters. Or as Mr Biggs puts it, "Don't I know you from somewhere a long time ago?"
 * George Clinton has a habit of recycling his older melodies in new contexts while making it seem more like creative fun than lack of originality. For example: The Funkadelic song "Red Hot Mama" borrows the melody from their older song "I Bet You", and the Parliament Song "Do That Stuff" takes a riff from "You Can't Miss What You Can't Measure" by Funkadelic.
 * The Thompson Twins' "Love on Your Side" has the line "I've played you all my favorite records" followed by a short recap of the melody line from an earlier single of the band's "In the Name of Love" (They felt it was the only one they could legally use).
 * David Bowie's "Ashes to Ashes" revisits the main character of "Space Oddity", Major Tom. The Major is also mentioned in the Pet Shop Boys remix of "Hallo Spaceboy".
 * Bon Jovi makes a nod to Tommy and Gina, the characters from "Livin' on a Prayer", in their song "It's my Life".
 * Queen have a b-side called 'Soul Brother' that's pretty much entirely continuity nods; "He's my best friend, he's my champion, and he will rock you, rock you, rock you"... it goes on like that.
 * "Hey, Mr. DJ, I Thought You Said We Had A Deal" by They Might Be Giants features numerous continuity nods to various TMBG in the third verse.
 * Even classical music features the occasional Continuity Nod by composers to earlier works. Just to name a few examples:
 * Robert Schumann's early piano suite Carnaval features, in the movement entitled "Florestan", a quote from his suite Papillons, written two or three years earlier. The autograph manuscript (and most published editions) lampshades this by inserting the word "(Papillon?)" in the passage containing the quote.
 * Dmitri Shostakovich's String Quartet No.8 is something of an autobiographical work; as well as featuring innumerable quotes of his musical monogram (D E-flat C B = D Es C H (German note names) = D.Sch. = Д.Ш., Shostakovich's initials), it quotes the opening bars of his Symphony No.1, the "Jewish" theme from the finale of his Piano Trio No.2, the opening bars of his Cello Concerto No.1, and an aria from his opera Lady Macbeth of the Mtsensk District among others.
 * The String Quartet No.11 of Shostakovich's fellow countryman Nikolai Myaskovsky features quotes from numerous songs and piano pieces which Myaskovsky wrote earlier in his career; the quartet is sometimes subtitled "Reminiscences".
 * The Russian Romantic composer Anton Rubinstein wrote three sonatas for violin and piano. The third sonata, written more than twenty years after the other sonatas, begins by alternating sustained notes in the piano with the opening phrases of the first and second sonatas in the violin before moving on to new material.
 * In the aptly-titled Four Last Songs, Richard Strauss included a musical quotation of his work Death and Transfiguration, which he had composed sixty years earlier.
 * Gilbert and Sullivan indulged in this on a few occasions. In their seldom-performed pentultimate collaboration, Utopia Limited, another nod is made to Pinafore as one of the British Flowers of Progress is a naval officer named Captain Corcoran - which is also the name of the captain of the Pinafore (for most of the operetta, at least). In his introductory song in the Act I finale of Utopia Limited, there is even a direct musical quote from the similarly-named character's introductory song in HMS Pinafore, "I am the captain of the Pinafore". It is somewhat uncertain whether they are intended to be the same character.
 * Utopia Limited also has a nod to The Mikado, specifically the title character's song about his Cool And Unusual Punishments:


 * Secondhand Serenade creator John Vesely references the chorus of his song "Your Call" in the first verse of the prior track, "Vulnerable" and even lampshades it, saying "...Isn't that a song already? I get a B in originality."
 * The Red Hot Chili Peppers make references to a girl called Dani in the songs "Californication", "By the Way", and "Dani California".
 * In Usher's music video for "Hey Daddy(Daddy's Home)" there are scenes with a group of female back-up dancers rehearsing. His following video "OMG" brings back the same dancers in full coustume performing the rehearsed moves.
 * The Faith No More song "Last Cup Of Sorrow" features the lyric "Raise your glass and let's propose a toast to the thing that hurts you most," almost identical to a lyric in the song Bloody Mary from an early demo by FNM singer Mike Patton's first band, Mr. Bungle.
 * Yusuke Nakamura occasionally does this with Asian Kung-Fu Generation's album art.
 * The wire on the cover of Houkai Amplifier connects to the string on Kimi Tsunagi Five M.
 * The guy on the cover of "Solanin" is wearing a Fanclub T-shirt.
 * There's a doll dressed up as the girl on Sol-fa on the cover of "Maigo Inu To Ame no Beat".
 * There's a Houkai Amplifier CD on Magic Disk's album art. If you squint, you can tell that the boy on the cover has been replaced with Gotoh.
 * "The Joker", by the Steve Miller Band: "Some people call me the Space Cowboy, yeah / Some call me the Gangster of Love..."
 * In an interview, songwriter Dennis Linde said that the Earl whom the Dixie Chicks kill off in "Goodbye Earl" is the same antagonist as in the much earlier "Queen of My Double Wide Trailer" by Sammy Kershaw. In the former song, Earl takes the narrator's woman off, and he takes her back.
 * Clint Black's 1994 single "No Time to Kill" makes several callbacks to his 1989 single "Killin' Time".
 * Keith Urban did this with two songs in a row: "Put You in a Song" is a Heavy Meta about how he wants to write a song about his lover. "Without You" has the line "And up until you came along / No one ever heard my song / Now it's climbing with a bullet".
 * Zac Brown Band created a fictional character called "Floaty Boatwood" for the video to their tropical-themed "Toes". Floaty shows up again in the video for the similar "Knee Deep" (a Jimmy Buffett duet).
 * KMFDM's "Oh Shit", the last track on Don't Blow Your Top, reprises a lyric from the title track, as well as reprising the instrumental of "Oh Look".
 * In 2010, country singer Lee Brice had a hit with "Love Like Crazy". A year later, he co-wrote the Eli Young Band's "Crazy Girl", which has the line "I love you like crazy, girl."
 * In Fear and Faith's 2010 song "Bought The Ticket, Took The Ride" references their previous album Your World On Fire with the lyric "watch my world caught on fire!"
 * ACDC mentions the title tracks of their first two Australian albums, "T.N.T." and "High Voltage," in "Dirty Deeds Done Dirt Cheap."

Newspaper Comics

 * In early Dilbert strips, the now infamous Pointy-Haired Boss did not have pointy hair at all, but rather just an ordinary, receding hairline of black curly hair. A 1995 strip depicting the slow decline of Dilbert's health benefits, via a series of flashbacks, depicts the boss with his old, unpointy hair.

Other

 * During the line to get in The Simpsons Ride at Universal Studios, various clips from the Simpsons are shown on TVs. One of them is a clip of Doc Brown from Back to The Future selling the rights to the time travel compound to Professor Frink.

Pro Wrestling

 * Throughout the Attitude Era, the WWF gave us a subtle Continuity Nod on a weekly basis, in the form of Chyna's nickname, "The Ninth Wonder of the World". You may be saying, "Wait, aren't there only seven wonders?" Yes, but in the '70s and '80s, the WWF billed Andre the Giant as "The Eighth Wonder of the World", and thus Chyna was next in line after him.
 * And Zack Ryder was in line after her, as he was recently called "The Tenth Wonder of the World".
 * The WWF shows from October '99 to around August '00 paid a greater attention to continuity than usual thanks to the efforts of then head writer, the late Chris Kreski. Kreski made a habit of extensively storyboarding everything and keeping continuity charts. One specific continuity nod sticks out in a world where allegiances are constantly changing and old ones are often ignored. Shortly before Kreski took over, Summerslam '99 saw the climax of the feud between Test and Shane McMahon over Test deserving the love of Shane's sister Stephanie McMahon. Test won a "Love Her or Leave Her" match and the next night on Raw, Shane told him he was the better man and that they should be friends from then on. After Kreski took over, and even for a long time after Stephanie turned heel on her family and Test to marry Triple H, Test had Shane's back whenever he was needed. I believe that at least for a while, this thread even outlasted Kreski's run as head writer.
 * Subverted by WWE firing the continuity editor when he pointed out too many issues and never hiring a new one.
 * These days, WWE doesn't tend to make Continuity Nods referring to incidents from more than half a decade ago (see Fleeting Demographic Rule). This tendency does not apply to appearances by or even casual mentions of Legends (WWE Superstars who are no longer with the company but are still widely remembered), who will often be referenced by younger commentators who weren't even born when they were in their heyday! One commentator earlier this year casually mentioned Sky Low Low - a midget wrestler whom very few fans over the age of 40 are even aware existed!
 * The WWE either intentionally does this (such as HHH never referring to his loss to The Undertaker at Wrestlemania 17 during promos for their fight at 27) or completely forgets about some stuff because they weren't important enough at the time. CM Punk reportedly had to remind the writers during his feud with Randy Orton that he was jumped by Orton and his stable in Unforgiven 2008, making him lose without even entering his title match, and thus giving him a pretty valid reason to go after Orton for Wrestlemania 27.

Radio

 * The Big Finish Doctor Who audio drama "The Kingmaker" features a scene in which a stranded Doctor leaves a note for his future self to find. It is "later" mentioned to have been delivered by "some bloke with big ears and a Northern accent", an apparent nod to Christopher Eccleston. In that same story, Tom Baker provides a cameo in the form of a clip from a history documentary, presented as the Doctor's taped notes (not actually Tom Baker, but a very convincing impression by Jon Culshaw).
 * Another character with an accent similar to Eccleston's also gets in a "Fantastic". Big Finish: tweaking the limits of BBC licensing restrictions for over 10 years.
 * And the fifth Doctor audio "The Whispering Forest" features the Green Crescent medical symbol from "New Earth". At the end,  sarcastically asks if the Doctor is leaving Purity because it's "too green for you", referencing the final line of "Kinda".

Tabletop Games

 * In both Warhammer Fantasy and Warhammer 40000, references are usually sprinkled around for fans to find referring to all manner of things from the previous editions that may have been forgotten about, passed over or deliberately retconned. For example, in the Grey Seer novel for Warhammer Fantasy, Grey Seer Thanquol makes a passing mention to the "blasphemous Kweekul", referring to the 2nd edition "Realms of Chaos" sourcebooks, during the time of which Skaven were actually part of the Chaos forces, and specifically referring to a Skaven Daemon Prince used to highlight the "design a Chaos God" rules. Similarly, the 7th edition Warriors of Chaos sourcebook mentions Lothar Bubonicus and Werner Thunderfist, two Chaos Champions (Nurgle and Tzeentch respectively) who each ascended to Daemonhood. Both of them were warbands played by the writers of White Dwarf, and their progress was shown in issue #124.

Theatre

 * In The Drowsy Chaperone, the Man in Chair mentions that actress Ukulele Lil probably played the ukulele, "although she doesn't in this show." And then she does at the very end, which is technically outside the Show Within a Show.

Video Games

 * In Castlevania: Order of Ecclesia at one point Shanoah comments "I am the morning sun, come to vanquish this horrible night!" a reference to Simon's Quest and the day/night mechanic therein.
 * In Castlevania: Symphony of the Night, when Maria first mentions Richter, she imagines him as the game renders him. Alucard, though, remembers his last encounter with a Belmont - and imagines Trevor's sprite from Castlevania III: Dracula's Curse.
 * Ace Attorney Investigations goes out of its way to never specifically describe any cases in the first three games, probably to avoid spoiling it for people who play Investigations first, but the continuity nods are everywhere for the older fans. A major example is that  goes from ambiguously creepy to chillingly terrifying if you know that
 * The music in the Magmoor Caverns in Metroid Prime and in the PYR sector in Metroid Fusion are almost note-for-note the same as the Norfair music from Super Metroid.
 * Not to mention the Torvus Undertemple music in Metroid Prime 2 being taken from the red area of Brinstar from Super Metroid as well, and the Pirate Homeworld music from Metroid Prime 3 being taken from Crateria from Super Metroid.
 * Phendrana Drifts from Prime 1 was remixed in both Hunters and Prime 3. And of course, Prime Pinball. Let's just say recent Metroid games are remixing a lot of music - which is no bad thing, considering how awesome the soundtracks are.
 * In the beginning of Super Metroid you travel through the exploded ruins of Tourian from the very first game. You can even see the remains of Mother Brain's tank.
 * Even a number of power-ups from the original were kept in their same locations. The Morph Ball, that ceiling-mounted Energy Tank just to its west, the one in Ridley's lair surrounded by the fake floors, and, retroactively, the missile pack under the ruins of Mother Brain's jar, located first in Super Metroid and then again in Zero Mission. And this is just scraping the tip of the iceberg.
 * In Unreal II the Awakening, a tech tests your powered armor with a simulated deathmatch, then mentions he could sell tickets, maybe even get Liandri to back it. This is a reference to Unreal Tournament (which was released after this game, but takes place before in the universe's timeline) in which the gameplay and plot (such as it was) revolved around a blood sport run by the Liandri Mining Corporation.
 * The dark-blue-on-light-blue humanoid robot Mega Man is one of the most recognizable characters in all of video gaming. Mega Man Zero, the sixteenthish installment of the series, was really quite effective at getting the players' attention by coming out of a long timeskip After the End with La Résistance being slaughtered by degenerate dark/light blue robot cyclopes.
 * In Mega Man ZX Advent, the "Control Center" area is very obviously the bombed-out ruins of Slither, Inc. HQ -- the final area of the original ZX.
 * In an extension, Area F in ZX is covered in snow during Aile/Vent's visit; by the time Grey/Ashe get to it in Advent, it's become a snow-free scrapyard thanks to Vulturon and a recently-revived Model W. The Highway in Advent is likewise a part of Area D that borders water, and the Floating Island is a distant part of Area A brought off the ground by, you guessed it, Model W.
 * ZX Advent has quite a few of these, such as the model train you're send to recover in one Fetch Quest. It's identical to the trains used in various missions in the Zero series. Another mission has you recover various "artifacts" that are all from the original series, such as an Energy Balancer (from 6) and a Life Tank (from 5). Additionally, the Three Wise Men in the game are named after the three main roboticists from the classic Mega Man series (and one may or may not actually be one of them).
 * Phantasy Star IV, being the final game in the series, has several. There is a town that has a statue in honor of Alis Landale, heroine of the first game. That very same town sells expensive Alis-themed merchandise, including a replica of her sword, which can be used as a weapon for Chaz (it pretty much sucks, although it's needed for a sidequest). An optional dungeon is the wreckage of a worldship like the one PSIII takes place on. Rika and Rune are (for lack of a better word) descendants of Nei and Lutz respectively. On the outskirts of town, Aiedo has a bakery with its storefront underground. The player can also visit an optional dungeon full of Musk Cats and meet their chief, Myau. Of course, none of these things are elaborated on in any way, so if you didn't play the first game, it can feel like you're missing out.
 * A bigger nod is the mission in Phantasy Star Online where you have to travel through a tough dungeon run to visit a small cake shop, a nod to a sidequest in the first game where you pretty much did the same thing.
 * The cake shop appears again in Phantasy Star Zero and Phantasy Star Universe.
 * Phantasy Star Online had an occasional special limited-time event called Max Attack G. In the Phantasy Star Universe storyline, researchers unearth some computer code and load it into a virtual-reality system. It's the original game code for M.A.G., and PSU players are given the opportunity to run through PSO levels, meet PSO npcs, and find PSO weapons aplenty...all as a promo event that has no bearing on the main story.
 * In one level of Star Wars: Republic Commando, you can find a lightsaber on a ship that's been taken over by mercenaries. Upon seeing it, the player character alludes to Obi-Wan's "civilized weapon" line, then adds "Well, times have changed."
 * The King of Fighters '98 features a headless Goenitz sitting in the background of the Black Noah stage, with wires coming out of his neck. In continuity, Goenitz took out Rugal Bernstein's left eye (the character to whom the stage belongs).
 * Capcom vs. SNK has a ton of these (as would be expected of a Crossover game), with many of them being character-specific. For example, playing on a certain stage with Kyo and Benimaru results in Goro Daimon showing up in the background, a reference to the older King of Fighters games where your teammates would watch you fight from the background and cheer you on.
 * In Jedi Academy the player finds the statue of Darth Vader Luke toppled in Dark Empire to defeat a group of Dark Side Elite.
 * In Tomb Raider 3, it is possible to gain entry to a secret room in Lara's mansion, which contains artifacts from her previous exploits displayed in glass cases.
 * Also, in Tomb Raider 2, a different secret room can be entered, which, amongst other things, contains an Egyptian cat statue, which can be seen in the Egyptian levels of the first game.
 * The Crystal Dynamics-developed games contain several nods to the films, such as the title art, Lara's father Richard disappearing in Cambodia, and Croft Manor coming under siege in Underworld after Lara discovers a secret chapel beneath the mansion.
 * Monster Rancher games frequently contain references to monster species not present in that particular game, including those that haven't been seen in several games.
 * The Halo series is fond of making references to itself, to the point where several characters, organizations, etc. initially exclusive to the Expanded Universe end up playing major plot roles in the games themselves, most notably in Halo 3: ODST and Halo: Reach.
 * A few minor examples in Halo 3 alone include the Cortana "visions", which make several references to the very first Halo book The Fall of Reach; a message from ONI Recon 111 to Miranda Keyes, which marks the first time the Office of Navel Intelligence is ever directly referenced in the games; and Cortana calling the Master Chief by his real name, which everyone who read the books already knew.
 * In The Secret of Monkey Island, there's a hollow tree stump that you can try to enter, only to be told to "insert disk 23"...then disk 47...and then disk 117, none of which actually come with the game. Many players apparently took the joke seriously and called Lucas Arts' help line about the missing floppies: In Monkey Island 2: LeChuck's Revenge: LeChuck's Revenge, the player can actually call the hint line and ask about the stump joke, prompting the operator to mention that she's sick and tired of people calling and asking about the stump. In The Curse of Monkey Island, you can enter a tunnel on Blood Island, and come out in that same tree stump, complete with the very same background used in the VGA version of the original game. Guybrush refuses to pass through, however, since the hole is too small for him to squeeze through, and the forests of Melee Island are rife with hyper-realistically rendered jaguars.
 * Note that the joke is changed in the CD version. rather then referring to the disks, Guybrush just comments about a series of catacombs. When you can check the stump in Curse of monkey island, you're in, well...the Goodsoup family catacomb.
 * By the way, catacombs may also be a nod to Indiana Jones.
 * This same joke is echoed in some of Tim Schafer's games, particularly Grim Fandango and Psychonauts (where examining a tree stump will make Raz discover a tunnel that opens up to a system of catacombs underneath).
 * Also from Secret, when Guybrush introduces himself to the Lookout with "I'm Guybrush Threepwood, and I want to be a pirate!", the Lookout responds "You look more like a flooring inspector." In Curse, when a ghostly bride with a thing for pirates asks Guybrush what he does for a living, our hero hastily answers "Flooring inspector."
 * In the vein of the above-mentioned stump joke, Curse features another throwback to the first game: If the player repeatedly asks Guybrush to walk into the ocean on Blood Island, he will eventually comply...and wind up under the pier in Melee Island from the first game, seeing his past self as a corpse, having choked to death under water. This is in reference to a puzzle in the first game, where Guybrush has ten minutes to free himself and get back out of the water before he chokes.
 * A Continuity Nod is found in the Database description of the top-down shooter stages in Bionic Commando: Rearmed. It mentions that the stages "remind you of the war stories Super Joe used to tell you." Super Joe is originally from Commando, an arcade game that was entirely a top-down shooter, and is the character you rescue in the NES version of Bionic.
 * Zork: Grand Inquisitor is filled with references to earlier Zork and Enchanter games. For example, one of the Plot Coupons is a Cube of Foundation like the ones found in Spellbreaker. Also, at one point the game has the player going back in time to visit The White House, the house from the original Zork. And of course, there's the ever-present threat of grues.
 * Grand Theft Auto is fond of those. Both Vice City and San Andreas, which were released after GTA III but are set before them in the series continuity, include character nods to III. The San Andreas nod has some emotion behind it, as the Player Character from III makes a cameo with a woman he'll end up killing at the end of his own game.
 * The Lost and Damned expansion pack runs in parallel with GTA IV; several events from IV are referred to on the radio, and one character in TLAD is killed offscreen near the start by "some Serb bastard," because he was going out with the daughter of a Russian gangster - a mission that the player did indeed carry out as Nico in IV.
 * The Ballad of Gay Tony begins  All three characters from GTA IV   At the end of Ballad,.
 * Oh so many in the Legacy of Kain series. Thanks to time travel the protagonists frequently revisit familiar locations, so there's plenty of chances for these and few of them are wasted.
 * In Soul Reaver, one area contains an enormous skull toppled to the ground, which those who played Blood Omen would recognize as part of the first big dungeon that Kain goes through. Later games, with their use of time travel, has Kain and Raziel ending up alongside other big events in the series; for example, Raziel's slaughtering of the six Sarafan inquisitors in Soul Reaver 2 coincides with Vorador's attack on the Circle of Nine.
 * The basin room Raziel enters from the Timestreaming Chamber at the start of Soul Reaver 2, and the hallways of the Sarafan Keep are modeled after the building seen . Furthermore, in Soul Reaver, have been modeled after the originals,  Incidentally,.
 * Also, the earring Kain sports in his evolved form from Soul Reaver onward is the ring.
 * The climax of Defiance takes place literally minutes after the end of Blood Omen, so the final few levels are full of nods to the first game, including
 * Summoner 2 contains many Continuity Nods and Call Backs to the previous game. Luleva and Erho, minor characters from Lenele, both make short appearances as adults, as does Empress Sihua
 * Sonic Chronicles: The Dark Brotherhood contains a particularly obscure Continuity Nod. One of Amy Rose's POW attacks is based on her ability to see the future -- in her first appearance in Sonic CD, this was why she was involved in the adventure in the first place... but seems to have been completely forgotten until this nod.
 * The Very Definitely Final Dungeon of zOMG! chapter one is filled with these. The ruins of Gambino's Tower, Grunnies, and Labtechs all make appearances. Not to mention the fact that the story ties many of the older storylines. Other Continuity Nods exist as well, but this is the most extreme example.
 * In the first Ratchet & Clank game, at one point the duo are "hired" by Gadgetron to serve as the image for their new line of hoverboards. Ratchet then asks if they will receive a special discount only for the CEO to explain they have to be with the company for two years before the employee discount kicks in. In Ratchet and Clank Up Your Arsenal (which was released two years after the first game), if you still have a Ratchet & Clank saved game, you actually get said discount.
 * Being a follow up to A Crack in Time (which also had quite a few nods, in and of itself), the new comic series has already had several of these, varying in importance. For example:  the hoverboots, the semi-forgotten tether for the omniwrench...and, most heartwarming, Ratchet's belief that, despite being rather incompetent, the Galactic Rangers would lend a hand, when things went pear-shaped. And that's just the first issue.
 * The first two games of the Fatal Frame/Project Zero games have no connection at all, beyond the Camera Obscura, however both are referenced frequently in the third game, especially with the protagonist of the first game is for some parts of the game a playable character. Another playable character is the uncle of the twin girls from the second game.
 * This has become very common in the more recent Tom Clancy games. The Ghost Recon Advanced Warfighter series, Rainbow Six Vegas series, End War, HAWX, and Splinter Cell all contain slight references to each other, since they all take place in the same universe. The vast majority of the nods are usually quick name drops, but a notable exception can be found in HAWX, where the player actively assists the Ghost Recon team on several missions.
 * Briefing cutscenes in HAWX also have some video clips that look like they're taken straight from End War, and the SLAMS anti-ballistic missile system is obviously the initial phase of the worldwide anti-missile shield the Russians in End War sabotage to start World War III. You can also fly the fighter plane that provides air support for the JSF in HAWX, and it's even specialized for air-to-ground, although its stats make it perfectly capable of dogfighting.
 * When playing as the JSF in End War, the Ghosts make up your riflemen, whereas the EF get Rainbow.
 * Gradius ReBirth, despite its name, is more or less a prequel to the lesser-known MSX Nemesis arc:
 * The plot is set in cosmic year 6664, 3 years before the events of Nemesis 2.
 * The pilot's name is James Burton, who was the protagonist of Nemesis 2.
 * Venom, the Big Bad of Nemesis 2 and 3, appears as James's CO.
 * The powerup sounds are from Nemesis 2 and 3.
 * The Suikoden series is rather fond of this. In the 5th game the gladiator Shoon mentions a fellow gladiator who had his eyes removed because he was too powerful, making for unbalanced matches. He was still far stronger than the others, so he was shipped off to the Island Nations and never heard from again. This isn't mentioned again in the game itself, but fans of the series may realize that he was referring to Morgan, a blind martial artist you can recruit in the first game, which takes place several years after S5 chronologically (supplemental materials reveal that Morgan's ship was wrecked, which is how he escaped and ended up in the Scarlet Moon Empire instead).
 * Suikoden 5's plot itself is a continuity nod to the second Suikoden's characters background; namely Georg, Lorelai and Killey.
 * Suikoden 4 has a character that appear in the first Suikoden
 * In Suikoden 2 you actually can visit Gregminster (the first Suikoden central city) and meet some characters from the first game too.
 * While House of the Dead: Overkill is mostly self-contained from its parent franchise, it includes a few more subtle callbacks to past games in the series. For example, the in-game title for Agent G's theme song is Suffer Like G Did.
 * In Banjo-Tooie, Klungo is last seen walking off into the distance, saying he's going to quit being Grunty's minion and "find easssy desssk job, maybe make ssstupid gamesss..." So, that's just Self-Deprecation coming from Rareware, right? Actually, when Klungo returns in Nuts 'n' Bolts, he's managed to create a minigame called Hero Klungo Sssaves Teh World, which is indeed stupid.
 * At the end of the first game, you compete in a game show against the Evil Witch, beat her, roll the credits... then you need to fight her AFTER the credits. And if you don't have enough Jiggies to get to the top of the tower, get some more. By the way - this description is exactly correct for the second game too.
 * The island of Mira, which played a major part in the first Baten Kaitos game, was conspicuously absent from the prequel, Baten Kaitos Origins. However, near the end of the game, it's mentioned briefly (which probably created a small Continuity Lock Out for people who only played the prequel).
 * Kirby & the Amazing Mirror has the Smash ability, which lets Kirby "attack with... familiar fighting moves", specifically his moveset from his appearances in the Super Smash Bros series. Oh, and the enemy you get it from? Master Hand (who also ).
 * Kotomine remarks offhandedly about how Tohsaka will feel much better after being buried underground for a bit amidst a conversation that skims along the topic of vampires. Shirou isn't sure whether to take the comment literally or seriously. This little conversation gave rise to some Epileptic Trees suggesting that Tohsaka is the descent of a vampire.
 * The Persona series:
 * Persona 4 makes a few references to the previous game. A few of the major characters from Persona 3 make an appearance as minor characters in the game.
 * Additionally, Persona 3 makes a number of references to the first two Persona games; for example, the Kirijo Group used to be part of the Nanjo Group, the latter of which was the company owned by the family of party member Kei Nanjo/Nate Trinity.
 * The Answer, the follow-up to Persona 3 included in the Updated Rerelease FES, has several flashbacks. One is to when Mitsuru first discovered her powers. When she uses her Persona, a scientist standing by is amazed. This scientist has no given name, but he's voiced by Dan Woren, meaning it has to be Shuji Ikutski (advisor to SEES in The Journey).
 * Almost the bread and butter of Disgaea. For example, the Prism Rangers making a reappearance in Disgaea 2: Cursed Memories... and are terrified of demons because of what Etna did to them in the first game.
 * Deus Ex Human Revolution has a lot of Call Forward versions of these. Not all of them are related to the plot of either game.
 * Final Fantasy IV: The After Years has plenty of such moments with regard to Final Fantasy IV, from Leonora being, to Calca and Brina's Band attack that turns them into Calcabrina, to all flashbacks to the distant past being done in the same graphical style as the SNES version of Final Fantasy IV, including playable flashbacks in Porom's and scenarios.
 * World of Warcraft has an involved continuity, and so pulls out many continuity nods to previous games in the series and early, obsolete content. The human starting area contains five farms, the same number as you are asked to build in the same area in the human tutorial mission in Warcraft I. Characters from all three games appear as minor NPCs in WoW, although some give quests and some have become enemies. Heirloom items, special items that level up with a character and can be traded between alts, are usually references to desirable items from the original game. The most commonly referenced nod is "Hogger", a level 10 elite monster in the human starting area who is not only the first monster players need to team up for, but can be quite difficult to kill even then. These continuity mods extend to mechanics, as well: the final boss of the Ulduar instance in the second expansion, the Old God Yogg-Saron, works somewhat similarly to the Old God boss that appeared in the original game, C'Thun: the boss stays stationary in the middle of a circular room, and needs to be attacked from the inside by a small strike force before it can be damaged on the outside. Both fights are involved enough, though, that the strategies required are different.
 * Ace Combat has quite a few of these in subtle details; the series article proper lists some of them.
 * Metal Gear Solid has a fair few of these in the overall series. A musical one occurs in Metal Gear Solid 2: Sons of Liberty: get into an encounter, and occasionally you can hear what sounds like the encounter music from the first game.
 * In the first Jak and Daxter game, Power Cells were the "most important Precursor artifact you could find." In the sequels, they inexplicably disappeared... until Jak X, when they were used for the Turbo Dash races.
 * The reappearance of Steele Stadium in Backyard Baseball, removed from earlier installments.
 * The Legend of Zelda:
 * The Wind Waker: The Great Deku Sprout reappears as a grown Deku Tree, all the sages are referenced and have stained glass windows in Hyrule Castle, there's a portrait of what is quite obviously Ocarina of Time's Princess Zelda (her dress looks different from the Wind Waker one), Tetra's other lucky charm (Not the golden one...) is a shard out of a Gossip Stone, the statues of the three golden goddesses look exactly like their respective Oracles from the Oracle games and there's a badass statue of Ocarina of Time's Link in Hyrule Castle.
 * Twilight Princess has one with the entry hall in the Temple of Time (of the past). It's an almost exact replication of the one from Ocarina of Time - right down to even the background music. Additionally, almost all the wolf songs are lifted from the playable songs in Ocarina of Time, Majora's Mask, and The Wind Waker, in addition to the Sacred Grove having Saria's Song, from the former two games, as its background theme.
 * In Spirit Tracks, Zelda makes a comment about how her ancestors were 'good at waiting'. Considering that practically every Princess Zelda in previous games, has spent most of it locked up and/or awaiting rescue. Also, Hyrule Castle has references to The Wind Waker, from having a giant stained-glass window of herself in the throne room, right down to how every single guard in the castle is dressed like her best friend and hero, Link. And Niko, the pirate who nicknamed Wind Waker's Link "swabby", is still alive. Naturally, his house has Continuity Nods as well.
 * The Legend of Zelda the Minish Cap's opening gives a subtle nod to the opening of The Legend of Zelda a Link To T He Past, whilst also subverting it. LttP begins with Zelda calling for help from Link and his uncle. In MC, she visits their house to take Link out to the fair. In both, it is down to Link's uncle whether he does or doesn't go. The house itself is visually very similar to the house in LttP, situated on a similarly small hill with some grass growing on the front.
 * The Legend of Zelda Skyward Sword, seeing as it was released during The Legend of Zelda's 25th anniversary, is full of references to pretty much every game in the series.
 * Thunder Force V is set sometime after Thunder Force IV. The Rynex, the protagonist ship of TFIV from a faraway galaxy, is found drifting within Earth's Solar System and its advanced technology is utilized by Earth for massive technological improvement. This goes over very badly with the AI in charge of the project.
 * In the second Spider-Man movie game, there are feathers near the top of the game's second tallest building. Why? Because that's where you fought the Vulture in the first game.
 * Even ignoring the identity of the final boss, Mother 3 still has a good number of nods to Earthbound. The Friend's Yoyo and Real Bat being references to Ness's choice of weaponry is one, but one of the funnier ones is the various "useless" actions the can take, which area ll basically the same as the useless "attacks" that Pokey would take when he was on your team in Earthbound, but with "Mechanical" appended somewhere. Like "Mechanically pretended to cry", or "suggested a mechanical truce" or "flashed a mechanically insincere smile".
 * This is the primary reason to load your save files from Mass Effect 1 to Mass Effect 2. Sure, there's a lot of important stuff that ports over, but almost every single decision that you made in the first game is referenced in some way. Truly, this trope is present to such a massive degree that the game feels completely different from if you just played it from scratch.
 * This is taken to an even further degree if you import your ME1-derived save of ME2 to Mass Effect 3.
 * At one point in Final Fantasy Crystal Chronicles: Echoes of Time, you encounter an area covered in poisonous gas. You are provided with a small pot that can be temporarily enchanted with a barrier that keeps the poison away, requiring you to carry the pot around with you if you don't want to take damage. People who played the original Crystal Chronicles will be very familiar with a mechanic like this.
 * he True Final Boss of Pokémon Gold and Silver and its remakes is Red, the hero of Pokémon Red and Blue, and he's highest level trainer in series history. He doesn't even speak, a nod to the fact that he was a Heroic Mime.
 * And in the Game Corner: "Behind this poster...is nothing!", alluding to Team Rocket's secret hideout from the first games.
 * The remakes of RBY (FireRed/LeafGreen) have a character talking about Misty having plans for a certain bridge as a dating location, referring to an actual dating scene from GSC.
 * There are other numerous references to previous games. A notable one, that typically passes past people, happened in Pokémon Black and White. One was the Hiker ferris wheel date, it features a Memetic Molester Hiker who seems to have an interest in the male protagonist. It relates to Generation 1 where there was a Hiker that would basically sexually harass you (the one with the Nugget), and you could only play as a boy.
 * In Rayman 3, if Globox is punched successively he will remark that Rayman was "nicer in Rayman 2".
 * Silent Hill isn't technically one cohesive continuity (except in the case of direct sequels and prequels), but several games still make references to each other:
 * The Good Ending to Silent Hill Origins attempts to answer the question of: "Okay, so just how did baby Cheryl end up by the roadside in the first place?"
 * Heather stumbles upon the old Silent Hill save points just before and gets to read Harry's thoughts.
 * Silent Hill 2 has a newspaper discussing the arrest of Walter Sullivan, a serial killer caught when he murdered a pair of twins; Walter's backstory is expounded upon in Silent Hill 4, and the twins have become terrifying in the meantime.
 * Lacking the actual location of Silent Hill as a setting, SH4 relied on a lot of Nods and Call Backs to tie it to the rest of the series.
 * It's hard to spot unless you're paying attention, but it also features the woods near a mysterious orphanage in Silent Hill and the mysterious tower from that orphanage as game locations - in this case, it includes copies of the information on the orphanage in case you forgot since Silent Hill 2... but it only hints at the in-game locations being those places.
 * Silent Hill Homecoming opens with a much older-looking Travis giving Alex a lift into Shepherd's Glen. (Thanks a bunch, Travis.) Arguably, Pyramid Head's Cameo can be seen as a continuity nod as well. *** FYI, Origins and Homecoming had the same development team.
 * Silent Hill, the Arcade machine, borrows monster designs from the other games, and the doomed steamboat The Baroness from Silent Hill 2 in particular.
 * Silent Hill: Shattered Memories: The UFO ending gives a hilarious Alternate Character Interpretation to the entire series:
 * For that matter, all of the games' UFO endings have nods to the other games: Harry shows up to abduct James in the second game's UFO ending, and then both of them appear in the third game; and  makes a cameo in the UFO endings of both Origins and Shattered Memories.
 * The Nancy Drew game series is packed with these, from the souveniers of past plotlines that turn up on Nancy's desk in the tutorials to newspaper and magazine headlines about celebrities she's previously investigated.
 * In Command and Conquer: Tiberium Wars the Red Zone missions include the withered husks of the Blossom Trees from Tiberium Dawn and Tiberian Sun. The first GDI Washington mission also has a statue of Nick "Havoc" Parker from the Renegade FPS.
 * One of the unlockable intelligence entries even quotes his negative opinion about the Mammoth MkII's retirement. It also confirms that he has made Colonel Badass Grandpa in the intervening time.
 * Plus, in some maps there's wreckage of humvees and Obelisks from Tib Dawn, and Nod powerplants and the wreckage of the aforementioned Mammoth Mk II from Tib Sun
 * In Sam and Max: The City That Dares Not Sleep, Sam recalls all the molemen he's met, including Shuv-ool and Dug from Sam and Max Hit The Road.
 * In the same episode, Max has an entire shelf of items from Sam and Max Hit The Road in his Inventory, including a Snuckey's pecan log, a bucket of fish, a Car Bomb game, and the roach farm from Sam and Max's office. The Flaming Max head in the room also speaks with a stereotypical New York accent similar to the one affected by Nick Jameson to voice Max in Sam and Max Hit the Road, a reference which is lampshaded if Sam examines the bucket of fish.
 * In the preceding episode, Beyond the Alley of the Dolls, if the player orders Max to use Psychic Ventriloquism on the jukebox in Stinky's Diner, he warbles "I remember my childhood in Brighton", the first line of Conroy Bumpus' big musical number in Sam and Max Hit The Road.
 * The team have Jessie James's hand mounted on the wall, an appendage used as one of the combinable items in Sam and Max Hit The Road.
 * In The Mole, The Mob and The Meatball, one of the nonsense things Sam can ask Bosco for is 'stray tufts of sasquatch hair', a reference to Hit The Road, where tufts of sasquatch hair were the MacGuffins used to locate the missing sasquatch.
 * Canvas 2 has multiple references to the original game and several of the heroines, mostly in Misaki and Saya's routes. Both of them are sisters of heroines from the original.
 * In Mario & Luigi: Bowser's Inside Story, there's a Shroob from Partners in Time sitting in the Fawful Theater.
 * After finishing the game, you can actually fight two Shroobs in the theater. The characters even reference events from Partners in Time.
 * Main series Mario games generally avoid using Continuity Nods, but Princess Peach's Castle has had the same general design since it appeared in Super Mario 64.
 * Paper Mario games, on the other hand, tend to make quite a few continuity nods... Even if they have to break the fourth wall to do it.
 * Golden Sun: Dark Dawn contains a lot of nods to the GBA games. Remember the Dancing Idol? Takeru? The girl you had to save from being washed away after she had been turned into a tree? You will now.
 * The mute monk, the adorable baby pirate, the Philosopher's Stone, the, and the werewolves and their Fantastic Racism problems all come back as plot points.
 * Subverted where you'd expect it most, though. The in-game retellings of the first two games' storyline suffer from in-verse unreliable narration including casting Felix as a villain and not explaining the Fire Clan at all, making the latter look like freaky dragon people from nowhere.
 * In Resident Evil 0, you take a short trip through Birkin's laboratory facility from RE 2. In RE 3, Brad is killed by Nemesis in front of the police station, where he appears as a zombie in RE 2 on a New Game+.
 * In Medal of Honor: Allied Assault's Behind Enemy Lines mission, you rescue the pilot of the deceased G3 officer from the first game, as well as meeting La Résistance operative Manon Batiste. In the second level of that mission, you can find the wreckage of the Greta rail cannon, in the exact same area where Patterson destroyed it. The final mission of the game takes you back to Fort Schmerzen.
 * The 2010 game has Sgt. Jim Patterson, the grandson of Jimmy Patterson.
 * Dead Space ends with  Dead Space 2 ends with
 * The Somtaaw Archangel-class dreadnought in Homeworld Cataclysm and the Hiigaran battlecruiser in Homeworld 2 share a similar design with two heavy ion turrets on the top and bottom. Seeing that Cataclysm is semi-canon and never gets a single reference from the sequel.
 * Touhou 10: Mountain of Faith introduces the immigrant goddesses and their Miko Sanae to Gensokyo.
 * Touhou 11: Subterranean Animism's Extra stage has Sanae reveal that Kanako was fascinated by technology from the outside world and gave the powers of nuclear fusion to Utsuho to spur a technological revolution in Gensokyo. This means Kanako is responsible for Utsuho's brief insanity that frightened Rin who set off geysers to call the heroines down to Hell to fight Utsuho, aka the entire plot of Touhou 11.
 * The geysers of Touhou 11 enabled the escape of the sealed-away youkai characters of Touhou 12: Unidentified Fantastic Object, making Kanako indirectly responsible for that game's plot, too.
 * Touhou 12.3: Hisoutensoku eventually shows us what exactly that technological revolution was for, and even includes stages in Story Mode where the characters enter a nuclear reactor in Hell and fight Utsuho, who is powering it.
 * That's four games so far the Moriya Shrine has been responsible for.
 * Another example is in Touhou 7: Perfect Cherry Blossom, when Alice greets Reimu and Marisa with familiarity, because she fought them in PC-98 exclusive Touhou 5: Mystic Square.
 * Heck, Marisa's "Master Spark" spell is a Continuity Nod, seeing as how she stole it from Yuuka in Touhou 4: Lotus Land Story, and her "Earth Light Ray" spell debuted in Touhou 3: Phantasmagoria of Dim.Dream (which is lampshaded when it's described as "a really old spell that she never expected to use again").
 * Another Lotus Land Story reference happens when Yuuka returns in Touhou 9: Phantasmagoria of Flower View, and Reimu immediately accuses her of being responsible for the incident because of how she was involved in "that other one". (which, incidentally, was pretty much composed of Reimu jumping random youkai for being youkai... which is also her storyline for Flower View!)
 * All in all, for a series that ZUN insists isn't a series, there are a lot of these.
 * In Gran Turismo, Special Stage Route 5 and Route 11 are set in the same semi-fictional city (based on Tokyo's Shuto Expressway system), and appear to have an overpass connecting them, resulting in rumors of an extra-long Route 16 track. This was partially realized with SS Route 7 in GT 5, which runs over said overpass and parts of the other tracks, as well as the suspension bridge (an analog of the real-life Rainbow Bridge) previously seen in R11's background.
 * At the end of Zelda: The Wand of Gamelon, Princess Zelda defeats Ganon with the title wand, and is for some reason trapped inside the Book of Koridai from Link: The Faces of Evil. again.
 * In The Godfather 2, your Made Man told to crack a safe might comment that he thought he would have to blow the lock. In the previous game, you had to blow safes open with explosives.
 * Star Trek Online takes this to positively ludicrous extremes, having references, shout outs, and nods to almost every place, thing, and character in Trek history, even a lot from the EU. Case in point, one of the NPCs who gives you a mission is a joined Trill named Damar Kahn, the seventh host of the Khan symbiont, who offhandedly mentions having also done research into wormholes (which is a reference to Lenara Khan (the sixth host)who appeared in an episode of Deep Space Nine).

Visual Novels

 * Shikkoku no Sharnoth makes numerous passing reference to Sekien no Inganock including mentioning the city of Inganock itself and a Runner known as David, who was a minor character there.

Web Animation

 * Homestar Runner has absolutely massive amounts of these. The most minor characters will pop up in various places, turns of phrase and vocabulary items will be thrown around in completely different contexts, objects from previous cartoons will make appearances elsewhere for little reason. At its greatest, a one-off joke will come back so much that it becomes a much larger part of the website, such as the band Limozeen. All you need to do is go to the Homestar Runner Wiki, find a cartoon's page and look at inside references, where each Continuity Nod, Shout-Out, and Mythology Gag will be documented in precise detail.
 * "Aw, shit. And I was in charge of confetti."
 * When Grif somehow revived Sarge from a gunshot wound he was chastised for how random that treatment was and Sarge ask if he would have treated a shot in the foot by rubbing aloe vera on his neck. When Grif talks to Doc a while later, a (not very good) medic, Doc tells Grif that he made the right decision and states that he just treated Caboose's gunshot wound in the foot by rubbing aloe vera on his neck (then his toe fell off).
 * A more recent one was in Recreation: Grif gets in the cockpit for a large vehicle and ask 'Why are there only four pedals if there's six directions?', the exact opposite of what Caboose asked when he first piloted his team's tank in season 1. There's been several per episode recently, though many are of the "blink and you'll miss it" variety.
 * Taken Up to Eleven in Red vs. Blue Revelations.
 * In Charlie the Unicorn 3, having been through everything else the cartoon is going to throw at him, at least he finds that kidney they stole back in the first one.
 * This fan work for Watchmen is filled with nods toward the original graphic novel, from Rorsachs' "love of animals" to the scene where the comedian falls out of a window.
 * Leet World: In "Satisfaction Guaranteed", Player has something of an obsession with a snowman's hat. In the second season, the same hat can be seen in the intro sequence.
 * In the Dragon Fable quest "The Risen", there's a nod toward the Mechquest side quest where you go to an old temple to look for fossils.
 * In Brain POP's Fractions episode, many of the robots from other videos on the site, such as Little Jimmy and the Cycloids from the Capitalization video and the Hillbot from the Country Music video, show up at Moby's birthday party.

Web Comics
"Belkar: The chick, we can sell into slavery. I know a guy who knows a guy."
 * Gunnerkrigg Court:
 * In chapter 13, Kat disagrees with Alistair about which is Prodigy's best album, then Alistair gives all his possessions to Kat before he leaves. In chapter 14, Kat is seen wearing one of Ali's T-shirts. Then in chapter 15, she wears a shirt with the XL Recordings (Prodigy's record label) logo.
 * In chapter 20, Annie accidentally leaves a giant fingerprint on the moon. Whenever the moon is shown in the background in later chapters, this print is still visible.
 * When they did a Bleach parody for VG Cats, Leo draws his Rat-Flail (an item he tried to make in a poor attempt at a D&D game by tying a rat to a stick), then turns it into an gigantic living Rat-Morningstar.
 * The Adventures of Dr. McNinja has its share of these as well. Some even link to Alt Text comments -- one memorable one being the Doc's List of Things To Do Before He Died. Another being the peg-faced pirates, and Dan McNinja's 'poison eyes'...
 * The Order of the Stick has plenty of these, acting as both tiny details and important plot pieces throughout the course of the comic. The most significant of which being an elder dragon who happens to be the mother of a young adult dragon the main characters killed over 400 strips earlier.
 * "Wait a minute, I had a 22!"
 * In the cast page, Haley is now holding an IOU note after she stole the diamond off the cast page.
 * "You're invisible!"
 * And then there's the boots that Haley wanted dyed when she bought her new armor. Remember? Those lime green Boots of Speed she found and mentioned to V way back when as what seemed like a one-off gag?
 * In strip #685, Buggy Lou mentions that Belkar "was pals with one of [his] suppliers" during Lou's "stint . . . buying slaves over on the Northern Continent." This is a reference to strip #171.

"Vaarsuvius: You are a bottomless pit of self-reference, are you not?"
 * And a Lampshade Hanging in strip #683 about a reference from strip #87:

"Hobgoblin Mother: You clean that spill up this instant, Hobgoblin Cleric #2! I need to go change your brother, Hobgoblin Warrior from Strip #443, Panel 3."
 * I think Rich just outdid himself in the 703rd strip:

""Man, I forgot that was there.""
 * An untitled webcomic frequently makes use this, frequently brining up often inconsequential side points from older comics and twisting their meaning and often connecting them to all kinds of other events. Recently it has been revealed that this is all the machination of the Illuminati of which Steve Jobs is a member.
 * Read the Alt Text on comic 576 of Xkcd. Now, go back and read comic 325.
 * Also in Xkcd, the super soaker ends up reappearing a while later.


 * Another Alt Text nod from Xkcd: Alt Text in #616 referencing #150. There's also an Alt Text referrence to the elliptical reflector dish.
 * Appears a few times in Something Positive. First is the reference to "I lava you," first seen on a note read by Davan and Nancy, and later repeated in a joke between Lisa and Gaspar.
 * There's also the time when Aubrey and Kestrel entered a supply closet in the Nerdrotica building, where Aubrey stored some props from her less successful escapades. She is shown picking up a Cthulu mask, a reference to an earlier plot where she attempted to film a show called "My Neighbor Cthulu."
 * One strip in a Christmas-themed arc in Brawl in the Family, has King Dedede starting to go around stealing things set to a Filk Song from How the Grinch Stole Christmas. Now, going back a year...
 * Matthew acknowledges that this strip is notably darker than standard Brawl in the Family fare, but check out the Alt Text and it'll point you to this one. Then all kinds of Fridge Horror pops up.
 * Unwinder's Tall Comics has a lot of these, sometimes resulting in Continuity Creep. For example, this comic references at least 5 different earlier strips.
 * Life and Death mentions the Gargle Blaster incident a few hundred pages later.
 * Many in The Inexplicable Adventures of Bob, with references to the Lint Mines of Dustworld, Bob's bottomless sock drawer, Molly wanting a pony for her birthday, the Grammar Squirrel, Molly's ridiculous fanfic crossovers (The Brothers Karamazov meet Harold and The Purple Crayon, or Gulliver vs. Mechagodzilla), etc.
 * Questionable Content's strip 1707 has a continuity nod from perhaps as early as strip 270, as Jeff spontaneously decided to draw Faye, Penelope and Hannelore with their old hairstyles. The nod may be to an even earlier hair style, if you count when Faye wore her hair with clips in colours other than red.
 * Made QUITE a few times in Triquetra Cats, which officially takes place in a probable alternate future of The Wotch, a short list includes the were creature jewel Wolfie and Katie use, (officially named in this timeline as the Samantha Stone, a reference to Samantha 'Wolfie' Wolfe) a descendant of Cassie Sinclaire named Circe, flashback panels showing Anne fighting Xaos, plus the plant that Cassie gives the love potion to is seen in a guest strip done by the Triquetra Cats team, this was later given a return nod in the Wotch where Glock mentions that "we need a full time force of people with expertise in mystical and scientific know how, and field agents prepped to deal with uncanny dangers, and entire SERVICE prepped to confront issues the public doesn't even believe in" in Triquetra Cats SERVICE is the name of the MIB type organization the main characters work for.
 * In El Goonish Shive, Grace's comment on The Princess Bride here is a reference to Susan's opinion of it here.
 * In Zokusho Comics Rotting Johnny assassinates a mage who was throwing around some powerful destructive magics. In a later issue, the Wayward Cross gets hired in place of the the mage to take out a group of goblins that have taken over a fort. At the end of that, one of the things they came to get from the fort is taken by the people who hired the Wayward Cross, so there may be further implications.
 * Homestuck is absolutely filled to the brim with in-jokes and references to past events. Often they become plot-relevant, but a lot are also just there for seasoned fans to pick out and to create a more cohesive work as a whole. (Andrew Hussie once claimed that there are very few pages that don't reference at least one other.) Strangely, there is actually an overriding plot reason for all of these, even the joke ones: circumstantial simultineity, in which similar events happen at the same paradox space time, is a real law of the Homestuck universe.
 * To cite one of many, many examples, in Act 5 Act 1 Vriska complains to Aradia that she may as well rip her heart out of her chest with her super strong robot arm and pound away with it, because apparently it's up to her to feel emotions for the both of them. Later on, Aradia, in a robot body, does indeed rip her heart out and pound it against something. Much later in Act 6, the Autoresponder tells Jake that the brobot might as well rip its heart out of its chest with its super strong robot arm and pound away with it, because apparently it's up to the artificial intelligence to feel emotions for the both of them; later, it does indeed do this. There is absolutely no real meaning to these nods, but they're sure as hell fun to find.

Web Original
"Jonah: * cough* Herpes * cough* Iron Man: What?!"
 * In an episode of I'm a Marvel And I'm a DC it featured Batman and Iron Man trading barbs about each other's movies. At the end, Batman proclaimed that Iron Man has herpes. After a year, Jonah Hex and Iron Man were trading barbs about each other's movies.

Western Animation
"Zoidberg: (trying on a Mariachi replacement shell) Hey gringos! Here comes El Zoido to poison your drinking water! Bender: Hey, I'm Mexican and I find that offensive!"
 * Chowder has one scene where Chowder is being interrogated by the police, and he breaks down and begins confessing to everything. This includes a reference to a previous episode where Chowder's brain becomes huge, realizes he's in a cartoon, and turns the show into a talk show.
 * On Batman Beyond, the Batcave is filled with relics from Bruce Wayne's crimefighting career, in the original Batman the Animated Series. This merges with Chekhov's Gun when Bruce uses Mr. Freeze's cryogenic gun to subdue a villain when she invades the cave.
 * In the same show, there is an odd example of a Mythology Gag becoming a sort of retroactive Continuity Nod: the elderly Bruce Wayne's pet Great Dane is named Ace, which was also the name of the Bat-Hound, Batman's pet dog who appeared in his comics during the 1950s and 60s. In a later episode, though, we learn about one of his later adventures as Batman, and the name of "Ace" takes on a whole new meaning:
 * And that episode was also filled to the brim with Continuity Nods, considering it was intended to be a Grand Finale. A visual that referenced the first episode of Batman the Animated Series, connecting a Joker plot in Justice League to the Batman Beyond movie, the random appearance of a villain we haven't seen or heard from in over a decade, the list goes on.
 * Terry McGinnis goes to Hamilton Hill High School, named after Gotham's mayor in Batman the Animated Series.
 * One of these relics actually makes a pretty good explanation for the unexpected reappearance of a certain character at the end of Return of the Joker. Even though said reappearance was a last minute decision. . This could also be seen as unintentional continuity nod.
 * The Splicers, people who use Lego Genetics as a form of body-art akin to tattoos & body piercing use technology descended from that used to create Man-Bat, as well as the various cat monsters in "Tyger, Tyger".
 * A very nice touch is the fact that the relic given pride of place in the cave (apart from the suit rack) isn't a trophy from a case, it's the costume of The Gray Ghost, Bruce Wayne's in-universe inspiration for his vigilante career.
 * South Park occasionally makes references to the events of past episodes. The 200/201 two-parter is especially rich on them, making it a perfect Milestone Celebration.
 * Bill Cosby (a robot from the future, It Makes Sense in Context) wants to be Cartman's friend in "Trapper Keeper". Cartman responds that he shouldn't have friends that are over 30 as he kind of got screwed on that one, a reference to "Cartman Joins NAMBLA".
 * "Free Willzyx" focuses on getting a killer whale to the Moon (as the boys are tricked into believing the Moon is their natural habitat). The whale is shown dead on the Moon's surface during the credits. Several series later in "201", Tom Cruise gets sent to the Moon where his corpse is displayed next to the whale's. The next series, both the whale and Tom Cruise's bodies are visible during the shot of the DP moonbase in "The Coon Part 2: Captain Hindsight".
 * Several episodes had nods to Kenny's deaths. Cartman's quote of "he dies all the time." in "Cartmanland", Stan saying "Who didn't see that coming?" after Kenny dies in "The Fourth Grade", then saying "That was a good one." in "Chef Goes Nanners" and treating it as unimportant when he was killed in "Underpants Gnomes", Timmy's likely knowledge of it after giving Jimmy a Kenny-style parka in the hopes that it would kill him, Kenny's mother giving birth to a new Kenny and her and her husband bringing up how many times this had happened, and Kenny's own knowledge of it when he is insulted that Stan treats Kyle's impending death more seriously than he ever treated his (just before he gets killed by a falling piano).
 * In "Awesom-O" Ms. Cartman says Eric is supposed to be grounded for trying to exterminate the Jews two weeks ago. He did just that in "The Passion of the Jew", which aired two weeks earlier.
 * The DCAU had tons of Continuity Nods throughout its run, the first of which is probably from the pilot of Superman: The Animated Series where Ma Kent encourages Clark to do an interview because she doesn't want people to think he's like "that nut in Gotham City."
 * In "Obsession", Billy Batson is clearly seen covering the fashion show (though Captain Marvel wouldn't appear until Justice League Unlimited
 * Plus the fact that the entire Cadmus Arc from Justice League Unlimited is essentially based off of the last episode from Superman: The Animated Series, and indeed, its conclusion refers to events as far back as the middle of the show's second season.
 * A classic from Batman the Animated Series; in the episode "Pretty Poison", Pamela Isley, (aka Poison Ivy), is dating Harvey Dent (the future Two-Face), intending to kill him for destroying a nature preserve to build Arkham Asylum. Later in "Almost Got 'im", Poison Ivy, and Two-Face meet in a Bad Guy Bar and start trading insults. Ivy then turns to Joker, Penguin, and Killer Croc and explains, "We used to date." (There's a bit of dramatic irony here in that Joker, Penguin, and Croc all respond with a casual "Ah....", probably thinking something along the lines of "she must have been a total bitch to him." They don't even realize that she almost murdered him!)
 * Then again, considering the kind of relationship the Joker has with his girlfriend...
 * In the Futurama episode "The Sting", Fry's funeral includes many guests from previous episodes -- including the fossilized Seymour, the women he had been with (including the radiator from "The Lesser of Two Evils"(It Makes Sense in Context)), and other characters and plot points. The DVD movies are just full of nods.
 * Futurama is basically one big nod. One of the best is one throwaway line in which Bender states that "bending" is his middle name -- meaning, of course, that he's good at bending. He then says that his full name is "Bender Bending Rodriguez" due to being made in Mexico, which any other show would never mention again and/or directly contradict in other episodes. Futurama, however, makes the name canon and mentions it often, even having an orphanage named the "Bender B. Rodriguez Orphanarium".

"Smithers: That's Homer Simpson, sir. All the recent events in your life have revolved around him in some way. Mr. Burns: Simpson, eh?"
 * In the episode "Lrrreconcilable Ndndifferences" Lrr asks Leela "Do I know you?" and she says "Yes I was in your mouth for about 5 minutes" referring to the episode "The Problem with Popplers" where she discovers the "Popplers" which are actually the Omacronian newborns.
 * In "Time Keeps on Slippin'", Fry tries to figure out what he did to win over Leela's heart (he can't remember due to time skips). Hermes suggests that maybe he's a fantastic lover; Amy quietly says "No.", referencing their brief relationship in "Put Your Head on My Shoulder".
 * In "The Late Philip J. Fry", the professor creates a time machine that can only move forward in time. His reasoning is so that "That way you can't accidentally change history or do something disgusting like sleep with your own grandmother." Fry, who did exactly that in "Roswell That Ends Well", seems to agree with this logic.
 * In many episodes of The Simpsons, there are small bits in the background which recall earlier episodes -- the enormous stone head that was in their basement for years, the electric hammer showed up in one of the Treehouse of Horror shorts, "Homer's Enemy" had Frank Grimes in complete disbelief when he saw Homer's Grammy award and photos from his time with NASA, and so on. However, despite these things, The Simpsons usually has Negative Continuity. It plays them against each other, like the episode where Homer took off Ned Flanders' roof to act as a snowplow, and Flanders complained that Homer used to be a professional snowplower and still had the equipment (and theme song). Similarly, Mr. Burns and Krusty the Clown never recognize Homer and Bart, respectively, even when all the previous episodes they've interacted in are mentioned.

"Bart: I'm Bart Simpson. I saved you from jail. ("Krusty Gets Busted") Krusty the Clown: Er, I... Bart: I reunited you with your estranged father? ("Like Father, Like Clown") Krusty the Clown: Er, uh, I don't know... Bart: I saved your career, man! Remember your comeback special? ("Krusty Gets Kancelled") Krusty: Yeah, well, what have you done for me lately? Bart: I got you that Danish. Krusty the Clown: And I'll never forget it."
 * In "Bart Gets Famous":

"Marge: Homer, I don't want guns in my house! Don't you remember when Maggie shot Mr. Burns? Homer: I thought Smithers did it? Lisa (quietly, with distain): That would have made a lot more sense..."
 * Earlier episodes featuring Sideshow Bob as the antagonist would typically include a brief recap of his past evil deeds.
 * In the movie, towards the end when Homer and Bart jump Springfield Gorge on a motorcycle, you can see an ambulance crashed into a tree - the same one that carried Homer away to hospital (well, almost did) after Homer failed to jump Springfield Gorge in more than 16 years earlier on a skateboard in the episode "Bart the Daredevil".
 * In the episode "Midnight Rx", the Canadian news refers to Homer as "Former American astronaut Homer Simpson".
 * In the episode "Homer the Heretic" Moe declares himself to be a snake handler. In the episode "Eeny Teeny Maya Moe" (aired more than 16 years later) he says that he's a "snake handler but not an observant one".
 * In the episode "Homer at the Bat", Smithers hires nine Major League Baseball players, including Mike Scioscia, to play against the team of the Shelbyville Nuclear Power Plant. All are prevented from playing by various misfortunes, with Scioscia getting radiation poisoning from working in the Springfield Nuclear Power Plant. Scioscia (now a manager for the Angels) appears in the episode "MoneyBART" (aired 18 years later); when Marge asks him about the radiation poisoning, he says it gave him "super managing powers".
 * In the episode "Much Apu About Nothing", Selma gives her full name as "Selma Bouvier Terwilliger Hutz McClure", referencing her previous failed marriages to Sideshow Bob ("Black Widower") and Troy McClure ("A Fish Called Selma"). It was, however, something of a revelation that she was ever married to Lionel Hutz.
 * Could be an Actor Allusion since Hutz and McClure were both voiced by Phil Hartman...
 * Maggie's shooting of Mr. Burns in season six is frequently alluded to in subsequent seasons, with the sheer implausibility of the incident often played up.

"Sokka: His [Pu Wan Tin] sources include singing nomads, pirates, prisoners of war, and a surprisingly knowledgeable merchant of cabbage."
 * In one Whole-Plot Reference to Flowers for Algernon, Homer is revealed to have been a genius as a child before he shoved a crayon up his nose. Many, many seasons later, during a growing-up montage (itself a Shout-Out to Amelie), in one photo his grade is an A+; the next, it's an F- -.
 * "The Heartbroke Kid" features The Itchy and Scratchy Show cartoon "Kitty-Kill Condition" where Scratchy visits Itchy the cardiologist. The questionnaire asks if he has ever experienced certain events such as being churned into butter and hung by his intestines, which he has in past episodes.
 * Also, the new version of the Simpsons intro contains more allusions to older episodes than the old version. This is especially obvious in the scene with Marge at the supermarket: the old version had her buying generic products, in the new version she can be seen buying Tomacco Juice (the plant was created in "E-I-E-I-D'Oh") and Mr. Sparkle detergent (first seen in "In Marge we Trust"). Also, she reads a magazine with an "Absolut Krusty" vodka advert (the vodka appeared in "Bart gets a Z").
 * "Politically Inept With Homer Simpson" featured a sign reading "SPRINGFIELD AIRPORT: Built For The Olympics We Didn't Get". In "The Old Man and the "C" Student" Springfield did apply for the Olympics.
 * In The Spectacular Spider-Man episode "Reaction" Spider-Man swoops down to save a nerdy-looking guy and an attractive woman, leaving them tied up together and looking infatuated ("You can thank me later, dude.") In the later episode "Gangland" (set on Valentine's Day) we see that same girl happily accepting the guy's marriage proposal.
 * Kim Possible does this frequently, especially during its fourth season. Not terribly surprising that the creators would want to throw out these little "treats" for the rabid fans, considering that they're the reason the show has a fourth season.
 * Just to give one of the many examples, remember Drakken's mind controlling shampoo? Hank Perkins does when naming Drakken's cupcake business. He mentions that "We tried "Dr D's", but people thought it was shampoo for some reason." It appears again, when Drakken later distils it for a more concentrated mind-control formula. Also, Warmonga - the alien who believes "the Great Blue" will lead her people - comes to Earth and sides with Drakken after picking up the TV signal of the American Idol parody that he appeared on to promote the shampoo.
 * In a season one episode of Duckman, the title character causes a supercomputer to go haywire by presenting it with logical conundrums. A season later, despite the show's usual Negative Continuity, he spills coffee on another such machine destroying it, and remarks "Wow, me and supercomputers, huh?"
 * In an episode of Rugrats, Angelica arrives at Chuckie's house and Chaz answers the door to which Angelica remarks "Nice to see your hair grew back" referring to the episode "Chuckie's First Haircut" in which Chaz allowed Chuckie to cut most of his hair off and was shaved bald by the end of the episode.
 * Xiaolin Showdown is always bringing back Shen Gong Wu (magical artifacts) from previous episodes. It has become impossible for all but the most dedicated fans of the show to determine which Shen Gong Wu changed hands when.
 * During its latest season, Foster's Home for Imaginary Friends has been making numerous continuity nods to previous seasons, such as when Mac saw the "Rocket Wars" action figures he was forced to sell online in a previous episode when he was at a swap meet.
 * WITCH uses it to the extent that each episode continues on into the next, and keeps drawing aspects from as far back as the first episode. Objects, items, dialog quips, and plot points are always being referenced and built upon, to the point where if you miss an episode you can become entirely lost. As a result, the entire series is a continuity nod to itself.
 * Used so often Avatar: The Last Airbender that it could have its own archive. But to mention a few: The umbrella from "The Fortuneteller" is found in Appa's luggage by Sandbenders in "Appa's Lost Days". The eye-patch wearing Fire Nation commander from "Jet" shows up again in "The Cave of Two Lovers". The tsungi horn and ruby encrusted monkey statue Iroh buys in "The Waterbending Scroll" appear several times. And so on.
 * And of course the Cabbage Merchant, who appears in three very seperate episodes and has his cabage wagon destroyed in some way. Referenced in the playbill in "The Ember Island Players," as are many other things (the play itself is a recap - kinda - of the series thus far).

"Edd: Have faith, Eddy. After all, you did teach Jimmy everything you know. Eddy: Oh yeah! Didn't we win an Emmy for that episode?
 * "The Western Air Temple" features the Gaang bringing up every single villainous thing Zuko did in the first season, and Aang telling them who the Blue Spirit really was.
 * Roku mentions that learning waterbending was particularly difficult for him, calling back Aang's troubles with earthbending, his own "opposite" element.
 * The entire finale of Codename: Kids Next Door features one nod after the other, framed around a scavenger hunt.
 * The numerous Fenton devices that make up Danny Phantom, not to mention several previous events that are stated in later episodes, "Memory Blank" has quite a lot.
 * Also, keeping watch during the Grand Finale could result in spotting such obscure ghosts as the trio from the comic book convention, the Danny clones, the Greek Mythology ghosts, the mutants that attacked the summer camp, and even the ghost that the Guys in White used to bait Skulker.
 * Danny: Hey, why is there all this meat down here?
 * The effective use of this in Aladdin and the King of Thieves is considered by many to be what made that movie better then the first sequel to Aladdin, The Return of Jafar, which entirely relied on the events of the first movie. That, and Robin Williams returning as Genie.
 * Teen Titans has several of these, including the baby moth-monster that Beast Boy keeps as a pet and Starfire drinking mustard as if it were a normal drink.
 * Ed, Edd n Eddy has a number of continuity nods:
 * Several scenes in the junkyard show inventions from previous episodes (like the swinging-chair ride from "Eds-aggerate")
 * In "To Sir, With Ed", Eddy convinces his friends to come over by telling them a snake had snuck into his room. They show up wearing uniforms for "Eddy's Snake Bee Gone", which are just the "Eddy's Hive Bee Gone" outfits from "Pop Goes the Ed" with "Hive" crossed out and replaced with the word "Snake".
 * Then there's this exchange from "Stuck in Ed", where Eddy convinces Jimmy to help him think up a scam, and Edd brings up the events of the episode "Ed in a Half-Shell":

Edd: Hardly..."

"Cleveland: I need to stop taking baths during Peter's shenanigans."
 * In "May I Have This Ed?" Ed produces a "lucky cheese chunk" he calls Sheldon Jr., a reference to his lucky cheese chunk Sheldon from "Thick as an Ed".
 * In "The Big Picture Show", Eddy's brother is shown wearing the same jersey and pants that Sarah and Jimmy wore in the episode "Ed, Pass It On", where they were masquerading as Eddy's brother.
 * In one episode, Eddy and Edd had to pull Ed out of Kevin's fridge, referencing an early episode where a sleepwalking Ed cleaned out every fridge in the culdesac.
 * The movie was full of continuity nods. References to previous episodes were shown throughout the episode.
 * The Venture Brother: watching through seasons 1 and 2 again after seeing season three, it's surprising just how much this series feeds back into itself like a giant ball of yarn.
 * Anything mentioned in passing in the show's numerous Seinfeldian Conversations will eventually get a nod. In Season 2, The Monarch mentions that when he first met Dr. Girlfriend she had red streaks in her hair; in Season 3's flashbacks, she does. In Season 3, The Monarch brags that he once tricked Captain Sunshine into thinking that he was invulnerable; when Captain Sunshine finally makes his first appearance in Season 4, he still believes it - and by the way, Captain Sunshine was mentioned as far back as Season 1, when The Monarch tells his henchman to return "the charred remains of Wonderboy" to him; he eventually admits that he "kinda slew" the sidekick, and Captain Sunshine is seen to still be searching for a replacement.
 * Camp Lazlo had a few of these. On the episode "Hello Dolly", Edward has a Barbie-esque doll named Veronica, who makes cameos later on "No Place Like Gnome" and "Beans in Toyland". On another episode, the campers start the Camp Kidney radio station back up, and Lazlo is shown in the station at the start of the episode "Samson Needs a Hug".
 * Family Guy has a few, but the most obvious one is when Brian throws a rock at Peter's head in the episode "The Fat Guy Strangler" (instead of hitting the serial killer who had just let him go). When Peter chastizes him he explains that he was aiming for Peter in the first place as belated revenge for a gag in the episode "To Love and Die in Dixie", which aired four years ago (specifically, not rolling down the window of their mock General Lee they had built before encouraging Brian to jump in through the window).
 * This seems to be a Running Gag with Brian. The episode "Airport '07" had Stewie about to take a drink from a cup that Peter had actually been using to spit his tobacco in. Brian started to warn Stewie about this, but then remembers a bit from an earlier episode, "Patriot Games", where Stewie beat him to a bloody pulp, and decides against telling Stewie anything (with unfortunate results for Stewie).
 * Cleveland gets quite a few nods in this direction, particularly in regards to the Running Gag of him always being in his upstairs bathtub when Peter's cutaway hi-jinks destroy part of his house:

"Stacy: Slow down! Remember how you thought Jeremy was with a girl, and she wasn't even a she? And then you ended up on that snowboard, all crazy and out-of-control?"
 * After Cleveland leaves Quahog, this same gag is revisited with Peter commenting about the empty tub: "Oh, that's right. Cleveland moved."
 * In the episode where Meg gets a legitimate boyfriend, every previous (and still living) character gathers at once to witness the 'miracle', including Cleveland who comments "This was worth the 500 mile trip!" (as his new show is set in Virginia)
 * The Kool-Aid Man crashing into the courthouse saying his "Oh, Yeah!" Catch Phrase during the first episode "Death Has A Shadow" was revisited again in the episode "Stewie Kills Lois".
 * In the episode "Deep Throats", Meg is dating Mayor West. In "Tiegs for Two" (aired five years later) they walk into a restaurant together, with Meg saying "it still happens sometimes".
 * In an early episode of The Tick, the villain Chairface Chippendale tries to write his name on the moon, but only gets three letters in. For the remainder of the series, every time the moon appears the letters "CHA" are visible on its face. The lunar abuse continues when the Tick goes to the moon to erase the letters, but only removes the "C" before being blasted into deep space. He meets a Galactus-pastiche and convinces him to spare Earth by snacking on the moon instead. A set of bitemarks accompanies the "HA" forever after.
 * It may be worth noting that this is something that shows up in the comics, too, as to this day, as the comics are still being published, every single panel with the moon in it will have "CHA" written on its surface.
 * In the pilot episode of Aqua Teen Hunger Force, a large rabbit robot (named the Rabbot) put a large hole in the side of Dr Weird's laboratory/asylum. The Rabbot-shaped imprint remained on the wall of his lab for the rest of the first two seasons and the movie, even playing a major role for how his creations escape or how leprechauns break in to steal his inventions.
 * The Aqua Teens' television is destroyed in nearly every episode. However, in "The Cloning" after Shake shoots the television with a flaming arrow, Frylock complains about everyone he destroyed throughout the series.
 * In an episode of King of the Hill, everyone is concerned about Hank's anger problem after he accidentally cuts off Dale's finger. Later on, when they go to a funeral of an anger management classmate of Hank's who had a severe anger problem, although dead, he still has an angry scowl, and Peggy says to Hank "that's what you look like when you sleep--" and Hank thinks back at all the mean things he's ever done shown in a montage of clips from many past episodes highlighting Hank's most aggressive and violent moments.
 * Nearly every time someone is shown reading a book it's always A Dinner of Onions, which is from the episode where Peggy buys a bookstore.
 * In a Season 5 episode, Buck wishes his son was like Bobby after he takes Bobby under his wing. Hank is surprised that Buck has a son, and Buck responds by saying that his name is "Ray Roy or something like that" and he lives in another state. In Season 13, when Buck meets his son in Memphis, he doesn't remember his name and calls him Ray Roy.
 * Invader Zim: One scene in "Battle of the Planets" has the Almighty Tallest recalling some of Zim's more humorous transmissions. The scene they were recalling -- "Remember that one time... when he called up... and he was completely covered with meat?" -- refers to the episode "Germs", which ended with Zim using germ-proof napkins to keep from contracting the titular threat.
 * In "Dib's Wonderful Life of Doom", aliens from the planet Meekrob give him special powers. Either a month before or minutes afterwards, depending on whether you go by airdate or production code order, Invader Tenn is said to be stationed on Meekrob in the episode "Megadoomer".
 * This episode also mentions that the Massive's weakness is its snack storage pods. In "Backseat Drivers From Beyond The Stars", the Resisty's attack on the Massive is to steal those snack supplies.
 * In "Battle of the Planets", Skoodge is seemingly killed when he is fired out of a cannon straight at Blorch, the Planet of the Slaughtering Rat People (It Makes Sense in Context.) However, he reappears in "Hobo 13", and rather than ignore the problem altogether Zim actually calls him on this. Skoodge Hand Waves it by saying he got better. This is, in itself, a nod to the first episode, where Skoodge is initially sent to conquer (or rather, be slaughtered by for being so short) the Rat People.
 * Dib goes to Zim for help in "Gaz: Taster of Pork", and Zim--being Zim--laughs at him and then turns him away. Dib mentions that he helped Zim when they were turning into baloney loaves, way back in "Bolognius Maximus".
 * Done a number of times on Phineas and Ferb, especially in later seasons.
 * In "The Chronicles of Meap", Dr. Doofenshmirtz mentions having to stand in for his family's lawn gnome, as he mentioned previously in "Lawn Gnome Beach Party of Terror!", and even tells Perry the Platypus "You remember that backstory..."
 * In "De Plane! De Plane!", Candace gets jealous when she sees Jeremy talking to another girl at the pool party, and Stacy references the events of "S'Winter" in warning her not to get carried away.

"Rigby: A buncha baby ducks, send 'em to the moon! Soda machine that doesn't work, send 'em to the moon!"
 * In "Invasion of the Ferb-Snatchers", Candace freaks out when she thinks Ferb has been replaced by an alien doppelganger, and goes to hide in the panic room first seen in "I, Brobot".
 * In "We Call It Maze", Isabella's mentoree Melissa recognizes Candace as the girl who earned fifty accomplishment patches in one day, as seen in the episode "Fireside Girls Jamboree".
 * Taken to the extreme in "Rollercoaster: The Musical!" "Mom, Look!" has Candace rattling off a list of the boys' previous schemes, and the finale "Carpe Diem" features almost every character in the whole show at that point.
 * In Bullseye, Linda mentions that lip synching isn't her only talent.
 * In Code Lyoko, all of the characters have their Limited Wardrobe overhauled right in the middle of William's possession, where he, being trapped inside the supercomputer, obviously would not have time to change clothes. His clone gets a new outfit instead...but when he's released from the supercomputer, he's still wearing his old outfit, as if to say "see, we remembered!" It might've been more effective if he hadn't come out of the computer, clothes changed, a few episodes earlier.
 * A similar nod with the flashback of how Aelita got virtualized for the first time years ago at the end of Season 2; she's seen wearing (logically) the same clothes as with her first virtualization at the end of Season 1. (Earlier flashbacks tends to have her wearing her current Limited Wardrobe clothes, but since those are basically Aelita hallucinating, they are a bit more subjective.)
 * In Stroker and Hoop, the private detective duo puts out a series of billboard advertisements that place them as the subject to ridicule by most who see them due to a typo. These billboards show up in a few episodes, even going as far as to getting them in trouble with a drug king-pin, who identifies them by the glaring billboards.
 * "Dammit, we still have those things?"
 * Every episode has in some way a background character with the same voice but never commented on. In the Series Finale they're captured by a guy seeking revenge against them but they don't recognize who He is. Turns out the voice was the same guy every time and goes on about Stoker and Hoop ruined His life, accompanied by a montage of His appearances.
 * Tiny Toon Adventures: "Buster Versus the Wolverine" features the Mynah Bird as a Running Gag.
 * In Winx Club season four, the final battle against the Wizards of Black Circle happens in the Omega Dimension, the ice prison Baltor escaped from in season three.
 * Not a very effective one, since there are no ice snakes present like last time.
 * Another trope undermined by 4Kids editing, and twice in a single episode to boot:
 * When the girls visit Gardenia in S2, Bloom sees auras over guards at a nightclub, in reference to Bloom seeing auras over many people in the S1 episode in which she visits her home on spring break. 4Kids cuts that part out. 11:21 in this video.
 * From the same episode, Vanessa talks to Bloom about having to testify on trial against the guys who burned down her flower shop back in the S1 episode in question (in fact, that's the whole reason she's headed to Gardenia in this episode), but in the dub, Vanessa talks about trying to get extra help for the flower shop. (10:08 in the above video.)
 * I must say, in S4, the writers did a good job remembering some of the magical abilities that Nabu had displayed in S3. (Then again, Nabu didn't have much screen-time in S3, and he only used his powers in three of the episodes he appeared in that season...)
 * In the SpongeBob SquarePants episode "New Leaf", you can see the briefly see the robot version of Mr. Krabs from "Imitation Krabs" in a pile with Plankton's other "failed attempts at the formula". It also reappears briefly in "Spy Buddies".
 * A rather subtle and interesting one in "Back to the Past". Spongebob and Patrick go back in time and see past versions of Mermaidman and Barnacle Boy using a giant can of tartar sauce to trap Man Ray. Now think back to Man Ray's first appearance in "Mermaidman and Barnacleboy III" where he is shown to be frozen in solid tartar sauce.
 * There was a rather pointless one in the sea whelk episode. When Spongebob learns that the whelks are sick, he exclaims, "I know! I had the Suds! I know what to do!" This was followed by something completely unrelated to said episode where he had the suds.
 * The newspaper at the beginning of the Wallace and Gromit short "The Wrong Trousers" bears the headline "MOON CHEESE SALES SOAR", a reference to "A Grand Day Out." In Curse of the Were-Rabbit,.
 * The Christmas Special Rudolph and Frosty's Christmas In July has continuity nods to many of the previous Rankin-Bass specials. Aside from Rudolph the Red-Nosed Reindeer and Frosty the Snowman (as well as their sequels, Rudolph's Shiny New Year and Frosty's Winterland Wonderland), reference is made to Santa Claus is Comin' to Town and The Year Without a Santa Claus.
 * Ruby-Spears' Mega Man was generally self-contained, but during an Enemy Mine situation, Wily comments on how nice it is that he and Dr. Light are working together again. This happened in both the games' backstory and the first episode of the show.
 * An episode of Jimmy Two-Shoes had Jimmy, Beezy, and Heloise singing a song in order to ger Samy to be distracted by his dreams of stardom in order to stop Lucius' plot. In later episodes, Jimmy's favorite band Runny & the Nosebleeds perform the song.
 * Some of the ghosts busted by The Real Ghostbusters can be seen in the containment unit in later episodes of the show.
 * G.I. Joe: Renegades will occasionally reference locations, incidents, and characters from previous episodes.
 * The Courage the Cowardly Dog episode "Mega Muriel the Magnificent" mentioned events that have happened in four previous episodes, such as "A Night at the Katz Motel", "Freaky Fred", "Queen of the Black Puddle", and "Snowman Cometh",
 * The Powerpuff Girls:
 * In "Nano of the North", the Professor's car has a license plate "PWR PROF", a reference to an earlier episode of the same name where he called himself "Power Prof" while wearing Powered Armor to fight crime with the Girls.
 * In the season 2 episode "You Snooze, You Lose", Mojo considers keeping the Girls in place on a Death Trap with flypaper, but brushes it off as stupid (instead of he uses chewing gum). Two seasons later, "Stray Bullet" has him actually using fly paper which completely works--so well that he's able to leisurely get a ridiculously huge collection of weapons and is still only stopped by a Big Damn Heroes moment from Bullet.
 * In The Smurfs: in the cartoon special "My Smurfy Valentine", Chlorhydris the evil witch spoke of Azrael having "royal blood in his veins" — which interestingly turns out to be true in Season 9's "Mummy Dearest" when the Smurfs meet his distant ancestor, the cat pharoah Azra
 * Three episodes -- "The Smurfbox Derby", "Handy's Window Vision", and "Skyscraper Smurfs" -- are continuity nods to things Dreamy has seen in a dream in "Gargamel's Miss-Fortune".
 * A second season episode of Regular Show is called "A Bunch of Baby Ducks". This comes from a line in the first aired episode, when Rigby explains he may have used The Power (a magic keyboard) to send things to the moon.

"Beavis: I know Daria killed herself, I remember that. Butthead: Uh, she didn't kill herself, she just moved away.
 * In Beavis and Butthead, during the "Ghosts 'n' Stuff" video in "Drones":

Beavis: Oh, really? Wow! That's kinda surprising."


 * Their latest meeting with Todd ("Snitchers") results in them having to testify against him in court. Todd's attorney then attempts to discredit them based on their stupidity, referencing "Frog Baseball" and "Held Back".
 * One Robot Chicken sketch was a promotional video for Cobra Command, and included Cobra soldiers being lectured to do things in a needlessly complicated and snake-themed manner, for instance instead of using a gas grenade they would use a gas-shooting robot snake in a wicker basket. A later episode in the form of a documentary about the battle over Cobra's Weather Control Machine ended with Duke saying Cobra Commander was captured then immediately broken about Zartan using "a wicker basket thing and a remote control snake or something".

Real Life

 * History Class.
 * In international diplomacy, it's customary for world leaders, when visiting another country, to make references to long-forgotten ties between their nation and the host country. For instance, when visiting Morocco, US Presidents will usually make reference to the fact that in 1777, Morocco was the first nation to recognize America's independence from Britain.
 * This occasionally gets to ridiculous extremes, like Turkish President Abdullah Gul's reference to "400 years of peace" between Turkey and Russia. Never mind that for the last 400 years, relations between Turkey and Russia are better described by...what's the word...oh, yes, war.
 * In 1890, the Turkish frigate Ertuğrul sank off the coast of Wakayama, Japan, after having an audience with Emperor Meiji. The surviving sailors were taken back to Istanbul by two Japanese frigates. In 1985, Turkey sent frigates to rescue 215 Japanese nationals who were living in Tehran at the time and endangered by the effects of the Iran-Iraq war. A statement released by the Turkish government mentioned it as returning the favor from 1890.