Reverse Cerebus Syndrome

So you're following this story. So far, its a very dark and edgy, tight and gripping thriller. Three main characters have died, the fourth is having an abortion in order to prevent the evil conspiracy from using her child to bring forth the apocalypse in a Crapsack World. Nobody smiles, ever.

Then you tune in next week, and find out that... the three dead guys have been resurrected thanks to the powers of a Magical Girl Bunny Ears Lawyer who takes the form of a polkadot unicorn. The evil government conspiracy is actually a sham by The Man Behind the Curtain, who just wants to impress chicks. The heroine's pregnancy was just a hallucination brought on by overcooked chocolate-chip cookies. The real villain is a demonic dog from hell who speaks entirely in pig latin. And there's No Fourth Wall any more, and the characters have a degree of Medium Awareness.

This Tone Shift away from seriousness and more towards humor can happen for any number of reasons. Maybe audiences weren't jiving with the more serious take. Maybe there was some Executive Meddling involved. Maybe the writers were just bored. In some cases, the story started funny, became serious, but then returned to its roots. And if it runs long enough, it may go serious again.

Inverse of Cerebus Syndrome, can be combined to get Cerebus Rollercoaster. Compare Lighter and Softer. See also Denser and Wackier when a series gets less realistic and zanier as it goes on. An instance of Mood Whiplash.

Anime and Manga

 * The original name for this trope was "Buu Syndrome," named after the Buu Saga of Dragonball Z, which brought the long-absent humor aspect of the story back in full force, featuring things such as Gohan trying to be a superhero only to be immediately found out, a tournament which quickly degrades into slapstick, villains who are initially so weak that the heroes aren't at all bothered by them, Goku getting favors from a god by offering to introduce him to Bulma, a sage who "brings out Gohan's potential" by forcing him to sit still while he reads comic books, and the world's most powerful coffee candy. To top it off, the villain of the story is a Cosmic Horror from the dawn of time who resembles a fat creampuff in Arabian Nights digs, who kills people by turning them into candy and eating them. While it did have some very dark parts, these chapters are generally so over-the-top that you can't help but laugh.
 * The Funimation dub of the anime took it even further by giving Buu a high-pitched Elmo voice.
 * So did the Ocean Group dub. It was provided by Scott McNeil.
 * Higurashi no Naku Koro ni had most of its 4-to-5-episode-long arcs slowly turning from amusing Slice of Life to gory horror. Then came the last arc of Kai, that was practically a shonen-style action-adventure story with Non-Lethal Warfare and.
 * Rei upped this with the funny variant, the first and the last episodes are sillier than any arc-beginning episode ever, and they don't even end in tragedy.
 * Kira is just straight-up Ecchi comedy.
 * While always humorous, earlier episodes of Sayonara, Zetsubou-sensei really presented the whole cast as deeply depressed and unhinged people, whereas later episodes rely on more self-referential and pop culture humor and the cast is generally better adjusted. For example, although it was always implied he never really wanted to kill himself, Nozomu's suicide attempts largely stop later on in the series. On the other hand, Chiri becomes increasingly murderous as the series progresses.
 * Code Geass underwent this likely as a side effect of Executive Meddling. Instead of continuing Zero's attempted hostile takeover of Japan after a genocide, the Reset Button is pressed via Time Skip and the show once again goes back to the wacky hijinks of the Absurdly Powerful Student Council. The English dub even hangs a lampshade on it.  Fans can only guess at what season 2 would've been like if the first several episodes hadn't essentially rehashed season 1, but it's been implied that C.C.'s real name (which was implied to be significant somehow, if only because they went out of their way to hide in in season 1) and the source of Suzaku's Charles Atlas Superpower would've been among the things revealed.
 * Yu-Gi-Oh! started as a surprisingly dark, bleak manga about a kid who had an evil alter ego that inflicted punishments on bullies that ranged from psychologically crippling, to killing, to eternally damning in limbo forever, but then that card game came along. Cue Lighter and Softer Plot Tumor, to which kid and alter ego start changing via The Power of Friendship.
 * To be honest, the post-card game manga wasn't that much lighter. It was still, after all, the manga where people could be amputated, blown up, used in black magic rituals or any number of things just for losing a card game.

Comic Books

 * The Savage Dragon started off as a byproduct of the Darker and Edgier period of comics. After about a year or so, the comics began to show some humorous characters, making the comic more fun. Once the 00's came around, the comic was turned into more of a Shout-Out to classic Marvel Comics and is a lot goofier.
 * Cerebus itself had this happen at least twice
 * The first instance occurred all the way back in Issue 3. The first two issues were - in all honesty - basically straight-up fantasy adventures that happened to star a Funny Animal and had some comic relief thrown in here and there. Issue 3 was where the series started to overtly parody the Heroic Fantasy genre rather than imitate it.
 * Later on, after years of progressively-darker and more-serious plot developments, Dave Sim gave readers Guys, Rick's Story and Going Home: three very comedic and light-hearted (although definitely not kid-friendly) story arcs.

Film

 * A major offender is the A Nightmare on Elm Street film franchise. The original movie was a very dark and spooky horror masterpiece featuring a mysterious and sinister Freddy. As the series went on, the plotlines became ridiculous, the deaths more over the top, and Freddy became more of a Large Ham who flung witty banter and product placement like nothing else.
 * The same could be said for the Friday the 13 th films, which became more tongue-in-cheek as the years went by, eventually ending In Space.
 * Leprechaun series began as a relatively straight horror... And then came parts four and five: Leprechaun 4: In Space and Leprechaun 5: In Da Hood.
 * Your Mileage May Vary. While it was slightly straighter than the sequels, the first movie had the title character making bad puns and riding a tricycle at superspeed. There was also a random kid sidekick in that one.
 * "F--k you Lucky Charms!"
 * The Child's Play series as well. While the concept of a serial killer in the body of a hybrid My Buddy/Teddy Ruxpin/Cabbage Patch Kid was never entirely serious, the earlier films were played much straighter than the later ones. In particular, the last two are definitely better classified as horror-comedies.
 * The James Bond film franchise. While there are still some dramatic character deaths, once it was out of The Sixties there was a lot more self-referential humour and lampshading of the Strictly Formula aspects. The Craig movies continue to play with the formula, but are far more serious.
 * The Godzilla franchise started off depicting the horrors of a nuclear holocaust. The first movie was very dark and even by today's standards, frightening. The series gradually went from a horror series to a children's movie series best known for goofy rubber suits and ridiculous plots. The title monster turned into a proud national icon and the source for cartoons, toys, video games, etc.
 * The Final Destination films can be accused of this. The first two films were genuinely dark and unpredictable with some nice Character Development and well written death scenes (except maybe for that one in the first movie). Then the third and fourth films basically upped the gore, nixed character development and basically became Bloody Hilarious.
 * The two Butter Cream Gang movies by Feature Films for Families experienced this. The first one was a straight drama about growing up, drifting apart from friends, and the pain in trying to keep said friendships intact. The second one did a complete 180 from that and was an adventure-comedy complete with buried treasure and inept mooks. Which one is better? It depends on what you're in the mood for.
 * The third The Neverending Story movie was widely criticized for having goofy depictions of characters like Falcor and the Rockbiter who spew pop culture references as opposed to the far more serious mood of the first movie.
 * This happened to the classic Universal Studios horror films: Dracula and Frankenstein and the Wolf Man started out in genuinely frightening movies, and wound up being outwitted by Abbot and Costello.

Literature

 * The first two stories featuring Retief were serious in tone. But by the third story, it had focused more on satire and humor, which the series is widely known for.
 * Dean Koontz's Frankenstein's series. The first two books are considerably darker than the last three.
 * Piers Anthony's Xanth series, especially the original trilogy, starts out serious and even shows more of the dark side of the author's imagination, and then becomes lighter and patently sillier, the smattering of puns (a few per volume) becoming a Hurricane of Puns, most of them cheap, and the content in general lampooning everything under the sun.

Live-Action TV

 * Angel went through something similar at the beginning of Season 5, although it'd be more accurate to say that the show suffered from permanent comedy-drama dissociative identity disorder. This was not new to season 5; the series radically shifted in focus several times, starting all the way back in season 1. The ending of season 5, however... not so much.
 * The X-Files underwent this in Season 6. For a while there, every single episode of the formerly dark, creepy drama was a comedy.
 * There had been a few comedy episodes here and there since late season 2, which was part of what made them work so well: they gave the viewer a brief, amusing break from all the darkness and edginess. The problem with season 6 is that the writers seemed to think the more of these episodes they made, the better it would be. The general agreement is that they were wrong.
 * The Fourth Doctor's tenure in Doctor Who underwent this shift. After a debut season that included such unsettling serials as "Genesis of the Daleks", seasons 13-14 had many violent horror-inspired storylines ("Pyramids of Mars", "The Seeds of Doom", "The Brain of Morbius"), to say nothing of the truly dark "The Deadly Assassin". Complaints from Moral Guardians poured in. When new producer Graham Williams took over for Season 15, the show quickly started to lighten up with the addition of Robot Buddy K-9 and the Doctor's humorous side had more airtime; in general, stories in this period were less gruesome and violent if not verging on comic ("City of Death" being the epitome of this). When John Nathan-Turner came on board as the producer for Season 18, he dialed back the humor and had K-9 written out of the show; the season ended with the Fourth Doctor's regeneration in the virtually humorless "Logopolis".
 * The contrast between the Sixth Doctor and the Seventh's Doctor's tenure is also thought of as this. The Fifth and Sixth Doctors had increasingly dark and convoluted stories involving parallel universes, and the Sixth Doctor in particular was batshit insane and more violent than most preceding doctors (perhaps except for some of the early stories with the First Doctor). Executive Meddling caused him to be abruptly replaced with the Seventh Doctor, who had much fluffier stories such as "The Happiness Patrol", which, while being a satire on Margaret Thatcher's Britain, is mostly remembered for having an evil version of Bertie Basset killing people with fondant.
 * Scrubs, which started off as a contemplative drama punctuated by zany comedic moments in the first season. Each successive season veered the show more and more into completely zany comedic territory with sillier and sillier hijinks and characters. Post-move-to-ABC, though, it's almost played straight, as the show turned back toward what it was the first season.
 * The first episodes of Passions tried to incorporate horror elements a la Buffy the Vampire Slayer or Charmed, leading to much Narm. Eventually, the show gave in completely to the witchcraft, complex death plots and total absurdity it's known and loved for.
 * Speaking of Charmed, the show zig zagged through this twice. Season 1 had many horror elements to it with demons such as werewolves, mirror ghosts and hell fiends. Cue the start of season 2 where the we have cupids, girls turning animals into men for dates and a bland love triangle as the main arc. This was fixed in season 3 but then season 6 went down this route again adding a surplus of fairies, leprechauns and woodnymphs as well as a nauseating episode with a magical "Mr Right". The seventh season fixed this.
 * Lost in Space began seriously, but the tone had given way to camp by the end of the first season.
 * While it had always been a highly optimistic comedy, How I Met Your Mother's first four seasons never shied away from emotional moments and tough, grim, even Tear Jerking storylines (Marshall and Lily's temporary breakup, Ted and Barney's fight, Ted getting left at the altar, Barney's unrequited feelings for Robin). Come season five however, it was all thrown out the window as everyone experienced Flanderization, the show underwent Denser and Wackier, and episode after episode revolved around pointless goofy filler. Even Robin and Barney's breakup had no emotional depth and no fallout until late in the season. The show reacquired some seriousness in season 6, however, and subsequently went too far in the other direction with extreme Cerebus Syndrome in season 7.

Newspaper Comics

 * In an overlap with Denser and Wackier, the comic strip FoxTrot started out relatively down-to-earth and realistic. There was no shortage of Story Arcs and Very Special Episodes; e.g., Peter trying to give up chewing tobacco, or Paige and Jason finding a hypodermic needle on the beach. Some story arcs took as long as two months, such as the 1997 arc where Jason attends summer camp. Sometime around the late 1990s-early 2000s, the comic became much looser and more comedic, often deconstructing comic tropes, breaking the fourth wall and exercising the Rule of Funny as often as possible. What little story arcs existed in the 2000s were often very off-the-wall, such as Jason dreaming that he's become a mini-Paige.

Video Games

 * Final Fantasy IX. After the angstfests of VI, VII and VIII, IX brought back some much-needed humor. It's still a dark game (its main theme is genocide), but it's significantly cuter and sillier than the previous installments.
 * Even before that was Final Fantasy V, which also had a much more lighthearted tone compared to its predecessor, Final Fantasy IV. Even though considerably more characters died for real in V than in IV.
 * Final Fantasy X-2 had a goofy, deliberately cheesy veneer masking a reasonably serious plot, whereas Final Fantasy X which was darker and grimmer most of the time.
 * Final Fantasy XIII undergoes a similar shift. It starts with an Inferred Holocaust, the main characters being Blessed with Suck against their will and being hunted down by the military. Oh, did I meantion they all hate each other? By the end
 * Team Fortress also underwent this change—compare the original Quake mod and its Valve Software remake to its sequel. The more realistic and less goofy-looking designs in Team Fortress Classic compared to Team Fortress, on the other hand, could be interpreted as regular Cerebus Syndrome.
 * Team Fortress 2 itself has been undergoing a variation of Reverse Cerberus Syndrome; while the story hasn't gotten less serious, the design process certainly has. When the game was launched, the art style stuck to a strict "Eagle Land during The Sixties" theme, and each character was boiled down to a series of instantly-recognizable traits, in terms of both design and game balance. Compare that to today, with over 300 increasingly-wacky unlockable hats and weapons inching further into Rummage Sale Reject territory. A Scout armed with a fish fighting a Heavy wearing a Dodgy Toupee was unthinkable in 2007, but is rather commonplace today.
 * Shadow Hearts An entire universe oozing with Eldritch Abominations is dead serious, then the second (third?) game halfway suddenly becomes lighthearted, and the third game is only terrifying if you actually pay attention to anything and read the in-game index for everything you encounter. Or otherwise do the side quests.
 * Despite how hard it tries not to, Diablo went through this. Strip out the names, and going from one game to the other, you'd not even know they were supposed to be related. The first is dark, slow, gritty, and actually downright scary at times. The second is somewhat brighter. The third game looks is far less like the first game and is much more colorful, though not quite to extent that Blizzard teased their fans over. (See the page image in Lighter and Softer).
 * As this article about Sonic Colors says, Sega aims to invoke this trope for the Sonic the Hedgehog series, starting with Sonic And The Secret Rings, after the Darker and Edgier games of Shadow the Hedgehog and |Sonic the Hedgehog 2006. Sonic Generations appears to continue in the same vein.
 * The Persona series went this route with Persona 4, which despite its plot about serial killers and, is just so darn cheerful and optimistic its signature color scheme is sunshine yellow, its mascot is a colorful and pun-spewing teddie bear, and it ends . After its predecessors' soul-crushing "Good" endings, and especially within the greater Megaten franchise, this was quite the Mood Whiplash.
 * The Command & Conquer Red Alert series gives us this. The first installment is fairly serious, with you either leading the Soviet Union's attempt to conquer Europe or the heroic Allied defense in an alternate World War II using mostly-realistic weapons. The second game features out-of-place technology (well, more of it than the first game), Psychic Powers, and attack squid. The third game includes, among other things, bear cannons and Tim Curry.

Web Comics

 * Saturday Morning Breakfast Cereal started as a very dark Black Comedy, then gradually evolved into Xkcd with more sex jokes, political jokes, and jokes based upon absurd science fiction and philosophy concepts. Compare this old comic with this one.

Western Animation

 * While Batman the Brave And The Bold, being the epitome of World of Ham, was always Lighter and Softer than Batman the Animated Series, Batman Beyond or The Batman, later seasons seem to be playing this up a bit more.
 * Slight in-universe example in The Simpsons: the Show Within a Show Police Cops is changed from an action series to a comedy and, to Homer's dismay, changing his namesake character Homer Simpson from The Hero to the useless Comic Relief.
 * The "Treehouse Of Horror" episodes, on their own, encapsulate this trope perfectly - the early ones were dark, murderous and were morbid at best and downright terrifying at worst (the one in which teachers are gradually eating through students is a perfect example). The modern ones tend to keep the bloodiness but have become much sillier.
 * The Simpsons itself is an example to an extent. It started with serious plots but eventually Flanderized it's self and it's characters.
 * The Land Before Time. What makes this obvious is the fact that all the films in the series except the first one are musicals.
 * Subverted in ReBoot's My Two Bobs. The first half of it features the return of S1's humor and Games as the main threat, which is far less dramatic than the previous Daemon Rising.
 * My Little Pony: Friendship Is Magic may be a borderline case when comparing the first two episodes to the rest of season 1. The unleashed Nightmare Moon and her desire to bring about eternal night represents a greater threat than pretty much everything else put together... and she's defeated by episode 2. The show then slips into a more everyday one-conflict-per-episode formula for the remainder of the season, until...
 * ... Discord comes along in the second season's first episode to flip it back by being much worse than Nightmare Moon... only to be defeated by episode 2.
 * The same occurred with the original cartoons. Originally there wasn't supposed to be anything after the first special but the series became so popular that they made another special, a movie, and a TV show based off said movie. The original special - called Escape From Midnight Castle - has little humor and is mostly just as much straight up action and Nightmare Fuel that they could back into such a short special. The next special, while still dark, is considerably more cute and lighthearted. The movie is mostly on-par with the second special but with more humor, and the series.. It's still dark but it's pretty fluffy.
 * Although the main plot of Beast Wars remained as dark as ever through the series, the third season played up much more slapstick than previous ones, and tended to exaggerate characters in general, resulting in a more surreal and comedic show.
 * The Flintstones started out as a t.v. show for adults until it was noticed it had a large kid following. It came on late and was even sponsored by a cigarette commercial.