The series has "The Joy", an episode which features a zombifying epidemic in the form of extreme happiness that reduces the infected to mindless smiling idiots who drool rainbow saliva. Grouchy Miss Simian is the only person left uninfected after the Joy claims everyone at Elmore Junior High and discovers a cure by accident—playing Beethoven's "Moonlight Sonata". She sneaks out with her tape player to try to reach the P.A. system in the principal's office, and after several near misses manages to get there...only for Gumball and Darwin to emerge from the darkness, grinning hugely, to infect her. She tries to leave a message telling the world the cure but is unable to sing the song or remember its name. The episode ends with Miss Simian smiling and drooling.
Subverted in "The Finale": Negative Continuity suddenly breaks down, causing all of the collateral damage, physical and verbal assaults, giant monsters, and other hijinks the Wattersons have caused for two seasons to spontaneously pay them back in pitchforks and torches. The Wattersons come up with a solution - cause even more chaos until Negative Continuity gets a jumpstart - but it basically fails and the mob actively tries to murder them. Seconds before they're about to be torn apart, Gumball laments that there isn't some kind of convenient, continuity-ending superdevice. Roll credits. Except of course, the cut to credits IS what officially fixes the Negative Continuity.
"The Flakers" ends with Gumball, Darwin, and Anais desperately scrambling to get things back to normal after they burn the kitchen and Richard (who is on anesthetics after a dentist visit) wreaks havoc across town (up to and including parking his car in a swimming pool). While they manage to do so by the time Nicole gets home and she is initially fooled, Richard’s anesthetics wear off and he describes his "dream" to Nicole, telling her everything that happened while she was away.
American Dad! had the Series Fauxnale "Hot Water" as this. In the episode Cee-Lo Green narrates this twisted tale of Stan buying a hot tub to relieve his daily stress, only to get into hot water when the hot tub begins killing his family and friends.
This happens in The Angry Beavers episode Fakin' It. Norbert spends most of the episode pretending that he was sick so he can avoid work and make Daggett wait on him hand and foot. But then, Daggett turns the tables on Norbert by pretending that he's sick. After Norbert gets angry at Daggett for tricking him, they both admitted to each other that they care about each other and made up until it turns out that they both got sick for real at the end of the episode.
Played for Black Comedy in the Bojack Horseman episode "Free Churro". Most of the episode is just BoJack giving a monologue about his now-deceased mother, whom he had a very rocky relationship with. Right at the end, the episode reveals that he'd been at the wrong funeral the entire time.
Being a Sadist Show with a strong streak of Black Comedy, Drawn Together has several of these endings. But special mention for this certainly deserves the ending of "Little Orphan Hero", where Captain Hero destroyed his home planet in revenge and raped his own parents to erase their memory. Played for Laughs, but still.
The series specifically references how cruel The Twilight Zone (1959) episode "Time Enough At Last" is by taking a parody of the episode to comically extreme lengths (e.g. his head eventually falls off) after which Bender comments that he was "cursed by his own hubris".
Seymore Asses, Fry's adopted stray dog in "Jurassic Bark". After an entire episode of Fry trying to resurrect his beloved dog, only to learn it lived to a ripe old age and died of natural causes. He concludes the dog lived a happy life and probably forgot him, and decides to leave it at that. Cue Flash Back during the credits that reveal the dog spent the rest of his life loyally and forlornly waiting for his beloved master to return, just as Fry had asked him to before ending up frozen. Even the writers felt this was needlessly sad and ultimately ret-conned it via Timey-Wimey Ball where there were two Frys thanks to time-travel, one of whom remained in the past as Seymore's owner.
“Do or Diaper” revolves around Mordecai being forced to wear a diaper if he doesn't kiss Margaret by midnight on Friday. As they’re about to kiss just before midnight, Margaret instead drops this:
Margaret: Have a nice week, diaper boy.
Mordecai, however, seems pretty happy despite this, since it proved he actually had what he needs to get into a relationship with Margaret.
One non-"Treehouse of Horror" episode that plays this straight is "Miracle on Evergreen Terrace". Its ending has the Simpsons returning home, thinking the Springfieldians have forgiven them. Instead, the town is there to repossess their belongings. The Simpsons are left with nothing to their names save for an old washcloth the mob missed, which they then proceed to fight over.
South Park has several of these endings, but the most prominent:
The ending of "Stanley's Cup". Stan's bike is impounded and to get the money he ends up coaching a pee-wee hockey team through the little leagues, and The Littlest Cancer Patient is waiting to see his team win a game. After drawing a first game, a second is scheduled for the interval in a professional one - but the opposing team fails to show up, denying the possibility of a victory. In a Genre Deconstruction of underdog sports movies, the pee-wee team is offered the opportunity to play the second half of the professional game against the Detroit Red Wings. To the cheers of the audience, Stan's team steps out against them. Of course, Stan's team are utterly destroyed, the cancer patient dies with "no hope" of seeing a victory, and the tone of the episode flips perspective, showing the Red Wings celebrating their victory as if they had been the protagonists.
Moreover, the ending of the episode "The Return of Chef". At the end of this episode, Chef was convinced of being a pedophile and then dies horribly burned and skinned by wild animals. However, the Super Adventure Club revived him and is transformed into a Darth Vader-esque cyborg who remained a pedophile.
"Scott Tenorman Must Die" mixes this with Karmic Twist Ending and Disproportionate Retribution. Scott starts making a prank on Cartman with his own pubic hair, then Cartman tries to convince Scott to give him back his money. When that doesn't work, note Scott burns the money in front of Cartman's eyes Cartman attempts several plans to avenge the humiliation, but every time he gets Out-Gambitted by Scott. Finally, after a very complex plan that involves Stan and Kyle trying to betray Cartman by telling Scott about what he tries to do, the latter's parents are killed and Cartman made chili with their bodies. And Scott ate the chili without knowing.
Parodied in Cartman's Christmas story from "Woodland Critter Christmas". "And they all lived happily ever after, except for Kyle who died of AIDS two weeks later".
In "The Bully", a new student at boating school starts picking on SpongeBob and threatens to kick his butt. SpongeBob tells Mrs. Puff about the bullying, but her actions do nothing to help him. Eventually, the bully begins to beat SpongeBob up, but since SpongeBob's a sponge, his body simply absorbs the blows, and the bully eventually collapses from the exhaustion of trying to beat up SpongeBob. Mrs. Puff then enters, sees the bully lying unconscious next to SpongeBob making a fist, thinks that SpongeBob beat him up and tells him that she's going to kick his butt.
"SpongeHenge" ends with SpongeBob escaping the jellyfish after ten years, only to find the Krusty Krab in ruins and believes Bikini Bottom was destroyed by a windstorm that occured in the episode. Although it does show the Krusty Krab being blown away by the windstorm before the Time Skip.
"Squid on Strike" ends with Mr. Krabs rehiring Squidward and SpongeBob after agreeing to all their terms of negotiation. This would be all fine and good if SpongeBob hadn't taken Squidward's earlier demands to "dismantle this oppressive establishment" a bit too literally. So when Squidward and Krabs arrive at the Krusty Krab, they find it literally dismantled (as in physically demolished) by SpongeBob. As punishment, an enraged Krabs then rehires SpongeBob and Squidward "FOREVER" as his eternal servants. Cut to "One Eternity Later", where Squidward's skeleton and SpongeBob's skeleton are sweeping the Krusty Krab floor.
In "Good Neighbors", Squidward buys a security system with the intention of keeping SpongeBob and Patrick out of his house so he can enjoy his Sunday off. When SpongeBob and Patrick's apology cake falls on it, the machine malfunctions and turns Squidward's house into a robot that destroys Bikini Bottom, and once it's shut down the three of them are sentenced to community service every Sunday for the rest of their lives to pay for the damages. SpongeBob and Patrick are excited to clean up the city with Squidward, but he looks like he's about to have an aneurysm.
"Gone" is one of the show's most infamous examples. When SpongeBob wakes up in the beginning of the episode, he realizes that everybody in town was gone. Eventually, he starts suffering Sanity Slippage. When everybody returns towards the end of the episode, they reveal that they were celebrating "National No-SpongeBob Day", where they leave town, build a wooden statue of SpongeBob, and set it on fire. Some people (such as Squidward) kick the ashes afterward. Even Patrick joined in on the holiday.
And in the episode What If... Thor Were an Only Child?, it seems to end quite happily. Thor cleans up his planet-wide party mess, sets up a date with Jane, and the Watcher comments how the people in this universe lived Happily Ever After... until a portal opens up with Ultron Drones emerging from it. And their leader? An Ultron who not only has the body of Vision, but also all six Infinity Stones. Even Uatu is taken aback by this shocking turn of events.
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