Production Throwback

The reuse of characters or items from a previous work in Easter Egg cameos in a newer one (similar to a fictional Production Posse or metafictional Continuity Cameo). In some cases, this lays down the basis of a Verse.

If it's something the actor did rather than the production team, it's an Actor Allusion. If the work in question is an unreleased earlier version of the same work, it's a Development Gag.

When this is done for works that haven't yet been released, it's Production Foreshadowing.

Anime and Manga

 * Mahou Sensei Negima is full of Shout Outs to Ken Akamatsu's previous series, Love Hina. Outside the numerous Expys, Tama the turtle has a cameo, the famous "Naru Punch" makes a reappearance, and one of Motoko's sword techniques sees some use in Negima. In addition, several characters from Love Hina make reappearances in Negima. The creator also confirmed that the hotel at which the characters stay in (one of) Negima's Beach Episodes is the same one where Naru and Keitaro stayed.
 * Akamatsu also confirmed that Nitta-sensei is the same Nitta-sensei from his first series, A.I. Love You. Not to mention that Negima's Big Bad Fate Averruncus is a rather obvious expy of Program Number 0.

Film

 * George Lucas films reuse the number 1138, after his debut feature, THX 1138 (which was itself derived from his phone number when he made the original student film version).
 * It's also used to access Easter Eggs on the Star Wars DVDs (push the buttons on your remote from the menu where you highlight the "THX" calibration logo).
 * This is also a common number to be used as an Homage (sometimes in credit to Industrial Light and Magic and the other Lucas production companies):
 * Matt Damon can be seen entering the last few digits of these numbers for the elevator access code in Oceans Eleven.
 * It appears on a door in big, obvious numbers in a shot practically set up just to frame it in Sky Captain and The World of Tomorrow.
 * In a similar vein, Stanley Kubrick would re-insert "CRM 114" into his later movies, after the name of the plot-critical device in Dr. Strangelove.
 * This also appears in homage on a label on some sound equipment in the opening scene of Back to The Future.
 * Likewise, Kevin Smith films reuse the number 37, after in Clerks.
 * And, of course, Jay and Silent Bob.
 * The DHARMA Initiative logo appears at the beginning of JJ Abrams' Cloverfield.
 * The Hanso Foundation, DHARMA's financial backer, is mentioned in the credits of Abrams' Mission Impossible III.
 * A DHARMA logo is also hidden in Abrams' 2009 Star Trek reboot.
 * An Oceanic Airlines advertisement appears in Fringe, another show by JJ Abrams.
 * Quentin Tarantino does this with brands like the Big Kahuna Burger.
 * The shared surname of the Vega brothers, Vic and Vincent.
 * Sheriff Earl McGraw from Tarantino's From Dusk Till Dawn, Kill Bill and both Grindhouse movies.
 * The Dude, She's Like, in A Coma rapist from Kill Bill re-appears as the guy with the car for the second half of Tarantino's later film Death Proof.
 * Machete, star of his own (no longer fictional) movie, shares the name, actor, and occupational field of a character from Robert Rodriguez's Spy Kids and nothing else.
 * Composer Mark Knopfler said that he would write the music for The Princess Bride only if Rob Reiner worked the hat he wore as Marty DiBergi in This Is Spinal Tap into it. Although he couldn't get the exact same hat, a similar one appears among Fred Savage's things around the head of his bed. (Knopfler later replied he was joking about the refusal to work on the film.)

Literature

 * Vermicious Knids, which originate from Charlie and The Great Glass Elevator, get passing mentions as monsters in a handful of other Roald Dahl books.

Live-Action TV

 * Any appearance of John Munch.
 * In Community episode Investigative Journalism Jack Black played a character named "Buddy Austen", who shares a last name with Jack Austen, the main character in the TV pilot Heat Vision and Jack (created by Dan Harmon) also played by Jack Black. Also,, made a cameo appearance in the same episode.

Video Games

 * Telltale's first game, Telltale Texas Hold'Em featured a mustached character named "Boris Krinkle", in which one possible line of dialogue has the character of Grandma telling him that he looks more like a 'Leonard Steakcharmer'." Naturally, when you first meet Leonard, sans mustache, in Telltale's Sam and Max episode The Mole, The Mob, and The Meatball, you get the option to say he looks more like a Boris Krinkle.
 * Hideo Kojima has a habit of inserting references to his previous works in his newer works, beginning with Snatcher, which included references to Metal Gear 1987 (such as Gillian's robotic companion modeled after the Metal Gear mecha), and then with Metal Gear 2 Solid Snake (which referenced the Snatcher Project and canonized Dr. Pettrovich's surname as "Madnar"), Policenauts (which included plenty of Metal Gear and Snatcher references), and the Tokimeki Memorial Drama Series (which included several Policenauts and Metal Gear Solid references). The most popular example is the Transplant of Meryl Silverburgh, originally a Policenauts character, into Metal Gear Solid.
 * Before creating Kingdom of Loathing, Team Asymmetric created a game called Krakrox the Barbarian. At least one item from that game appears in Kol, the Ring of Half-Assed Regeneration.
 * And there's also an item that lets you play as Krakrox for a few adventures.
 * And now Krakrox's Loincloth, "originally owned by the famous barbarian adventurer Krakrox," is part of the Seal Clubber's Legendary Regalia.
 * The arcade version of Double Dragon features the red sports car from Data East's FMV game Road Blaster (a.k.a. Road Avenger) inside Billy and Jimmy's garage, as well as a billboard advertising Nekketsu Koha Kunio-kun (the Japanese version of Renegade) just before the first boss battle. Both were games previously directed by Yoshihisa Kishimoto, the director of Double Dragon. In the arcade version of Double Dragon II, the helicopter from Cobra Command (Kishimoto's other FMV game he did for Data East) appears in the garage at the beginning as well.

Web Comics

 * MS Paint Adventures has jokes from earlier adventures in the same series as well as unrelated old webcomics by the same author. Thus, in Homestuck, you have allusions to pumpkins disappearing and retrieval of arms from the author's first experiments with the format.

Web Original

 * Anders Sandberg, one of the big contributors to Orions Arm has worked on several rpgs in the past, including Big Ideas Grand Vision. Every human colony from this game has been transplanted into Orions Arm, after being suitably altered to fit in with the new setting.

Western Animation

 * Pixar does this in most, if not all, of its feature films, usually with characters from its shorts:
 * The lamp from Pixar's (canonically) "first" short, Luxo Jr. crushes the i in "Pixar" in the studio's standard Vanity Plate.
 * The ball from the same short likewise appears in pretty much every movie they ever make.
 * Similarly, the Pizza Planet delivery van in every film since Toy Story.
 * The old man Geri from "Geri's Game" reappears as a toy repair man in Toy Story 2.
 * In The Simpsons, characters from Matt Groening's Life in Hell comic strip (usually Bongo) sometimes appear as stuffed toys.
 * In the early 1990s Simpsons arcade game, they appeared as enemies in the Dream Land level (as well as every interstitial title screen).
 * In Futurama, Bongo appears in a pet shop.
 * Sintel features characters from the Blender Foundations two previous shorts. In the market scene, you can clearly see Proog from Elephants Dream, and the butterflies in the bamboo forest are just a Palette Swap of the ones from Big Buck Bunny.