In episode "The Enchiridion!", this used as a Secret Test of Character. After Finn saves a trio of gnomes from a fire pit, they go on a sudden old-lady killing spree, claiming every action of Finn's as justification for the death of additional old ladies. Jake reseals them quickly and delivers a surprisingly straight Aesop (for this show, anyway) to Finn: aside from the fact that the old ladies were fake, the gnomes acting like jerks is in no way evidence that Finn isn't righteous.
Used again in the season 4 finale "The Lich". Finn collects all the crystals of power in Ooo so Billy the Hero can push the Lich into the portal the Enchiridion will open when the crystals are put inside. It turns out the Lich had possessed (and possibly killed) Billy, and the episode ends with Finn and Jake following the Lich into the portal, followed by what appears to be a shot of Finn and Jake in the mirror world seen in "King Worm". We don't find out what happened until the season 5 premiere "Finn the Human" and "Jake the Dog".
The Adventures of Jimmy Neutron, Boy Genius: 99 out of 100 times, Jimmy's inventions end up causing problems for everyone in Retroville, leading him to fix what he broke. As revealed in "Clash of the Cousins," the majority of the Neutron extended family views him as the Black Sheep as a result (at least until he saves them from the new Neutron black sheep: Eddie, his infant cousin). It's even lampshaded; in "The Incredible Shrinking Town", Libby remarks that Jimmy's endangered the town so often that she's just Stopped Caring.
Arcane: While Caitlyn had no way of knowing Powder would react poorly, her showing up caused the reunion between sisters Jinx and Vi to go straight down the drain, with Jinx assuming Vi was working with the Enforcers to arrest her.
In the battle for the Northern Air Temple, the Mechanist deploys a balloon to defeat the attacking Fire Nation forces, but the balloon crashes. The retreating Fire Nation forces take the balloon and are able to mass-produce balloons and zeppelins to augment their military.
The Gaang taking down the Government Conspiracy in Ba Sing Se may have felt right, but it was ultimately detrimental since it led to the nation destabilising and less able to hold their own against the Fire Nation.
The "Gaang" leaving to do their own things before meeting the Kyoshi Warriors. Otherwise, they would've realized that it was Azula's group instead and they wouldn't have learned about the invasion plan.
Then a few episodes later, Katara sees Zuko serving tea and instantly runs to inform the authorities. If she had investigated further, confronted him herself, or thought about the situation for more than a second, Zuko never would have been put in a position to turn to Azula's side to begin with. (And Aang probably would have finished his last chakra.)
In the episodes "Beginnings Part 1" and "Beginnings Part 2", Wan sees two massive spirits fighting and threatening to destroy a valley. One begs Wan to help him, while the other tells him to stay out of their conflict. Wan breaks the two up, but it turns out that the spirit that he freed was Vaatu, spirit of chaos and darkness, who goes on to start a massive war between spirits and humans and will bring about The End of the World as We Know It if he isn't stopped.
It appears we have another one at the end of season 3 with Tenzin telling Avatar Korra to just take her time and recover while he and the other airbenders keep the balance in her place. In trying to comfort Korra and support her in her time of need, Tenzin actually confirmed her worst fears that the world no longer needs her and the Avatar.
Season 4 gives us a Well-Intentioned Extremist variant with Zaheer. Politically destabilizing the Earth Kingdom may have felt right, but instead of creating the freedom he was hoping for, he instead allowed an even worse tyrant to take her place.
The premise of The Adventures of Puss in Boots involves the hidden city of San Lorenzo, no longer hidden because Puss-In-Boots took a coin out of the treasury. Dulcinea guilts him into staying to protect the city from all the thieves that will now come as he is the reason they need to be defended in the first place.
In The Adventures of Super Mario Bros. 3 episode "Seven Continents for Seven Koopas", the Koopalings take over Earth. What does Mario do to undo these actions? Turn the Great Wall into a dragon, and flood the Louvre.
In episode 2 of The Amazing Chan and the Chan Clan, the kids successfully rescue Boo Blew from kidnappers...only to discover they've nabbed a lookalike who didn't want to be "rescued" and blown his cover as a double for Mr. Blew.
In the American Dad! episode "For Black Eyes Only", the main antagonist's plan is to buy several tons of sand and thousands of hair dryers so he could melt an ice cap, then have the resulting water slightly flood Detroit so his home can become beachfront property. The only thing preventing the resulting tsunami from hitting the US is that he unplugs the hairdryers at the last second; Stan replugs them and the tsunami hits, resulting in half the US being underwater and millions of lives lost, including the president.
Quite literally in Season 3 when the trio overload the Omnitrix trying to get it to access more alien forms (granted it was well-intentioned). Which in a later episode leads into another case of this trope in...
"Time Heals". Gwen successfully goes back in time and prevents the messing with the Omnitrix that caused Kevin's mutation... only for her to find out that the change to history has caused a Bad Future where the trio were unable to stop Hex and now Hex rules the Earth, Kevin is Charmcaster's slave, Ben's chained in a dungeon as Hex's prisoner, and she's dead. It gets worse when you realize that, unlike some other examples on this page, where the hero had no idea what their actions would do, Gwen was warned by time-traveler Paradox not to go through with her plan. True, he didn't actually come right out and say what exactly would happen, but if a near-omnipotent time-traveler who can see multiple timelines and futures, and usually doesn't visit unless something is really important, tells you not to do something, you should probably listen. Of course, Paradox probably knew Gwen wouldn't listen, but that doesn't give her an excuse.
Ben's movement scans help the Stalker counter them.
Another example of breaking: Kevin accidentally releases P'andor, a highly-radioactive energy being, from his containment suit. This leads to an Oh, Crap! face.
A very notable case is that Ben and the others were unaware that they were unwittingly giving Aggregor back his captives each time they helped them. The last of the five finally pointed it out, and he too was caught. Just goes downhill from there.
In the Series FinaleA New Dawn, during time-traveling shenanigans, Ben's transformations inspire a young and then-exiled Vilgax to develop an interest in obtaining the Omnitrix's transformative power, making Ben indirectly responsible for everything Vilgax has done as an intergalactic warlord.
In the Bob's Burgers episode "The Deepening", we have Bob's plans to stop a slowly moving animatronic shark from an old movie. He tries a spike strip; the spike strip ends up wrapped around the shark spikes-out. He tries an oil slick; the slick makes it faster. He tries a hook and crane; the weight of the shark pulls the crane onto a power pole, electrifying the shark. Lampshaded by Hugo when he states that Bob's only making it stronger, to which Bob yells "he knows he's making it stronger".
Captain Sturdy: The Originals had Captain Sturdy making peace with his retired enemy Dr. Destructo, only for their little meeting to end with Dr. Destructo getting beaten to a pulp by Captain Sturdy's sidekick Ultra Boy and his old teammates the Originals because they mistakenly thought Dr. Destructo was up to his old tricks. Dr. Destructo at first seems to take the incident in stride, but then gloats that Captain Sturdy's allies assaulting him have inspired him to return to crime and dedicate the remainder of his life to destroying Captain Sturdy and every other superhero.
On ChalkZone, Rudy causes most of the problems that he needs to fix, whether it be because he left the ChalkZone portal open, he brought something from the real world into ChalkZone that causes a problem, or he caused some sort of problem in ChalkZone itself. Some episodes where this happens include "Gift Adrift" (though Snap made it even worse in that one), "The Wiggies", "Waste Mountain", "When Santas Collide", The Big Blow Up, "Draw and Let Draw", "Hole in the Wall", and "Power Play", just to name a few.
Code Lyoko's first season is a Strictly FormulaMonster of the Week series of random events in which XANA tries to attack the kids in the school, prompting them to go to the factory and hit the Reset Button. Later on, we (and the characters) learn that this actually makes XANA more powerful whenever they do it - meaning that the entire first season was the heroes powering up XANA to the point where he is actually able to attack them directly, except through taking control of electronics or objects that might potentially cause harm.
Cyberchase: In "Lost My Marbles", the kids were squabbling over how to use a library map. This allowed Hacker in Cyberspace to breach Motherboard's defenses and infect her with a virus that there's practically no way to cure.
In "The Snelfu Snafu: Part 1": The group enters an auction to win a copy of the Encryptor Chip that will fix Motherboard's virus. It turns out that the contest was actually set up by Hacker who created a fake Encryptor Chip. Once inserted into Motherboard's computer she is overwritten and replaced by Hacker, allowing him to take over Cyberspace for the first time!
Danger Mouse: The serial "Day Of The Suds" has Danger Mouse disposing of Baron Greenback's army of renegade washing machines but then gets hounded by a nosy TV reporter for not only disrupting London's need to do their dirty laundry but for the subsequent creation of a giant suds monster caused by the strewn detergent, the sparks from the damaged cables and the fuel used to run the machines during their rampage.
In "The Devourer", Zick ended up getting stuck in the Dom-Devourer, a device that is impenetrable. Not only that, for each zap, it saps his powers. By his continuous zap, he loses his ability to see and talk to monsters.
Darkwing Duck: In the episode "Star-Crossed Circuits", D-2000 going haywire and turning into a yandere results from a combination of Gosalyn showing it a soap opera and Darkwing upsetting a can of cola over it soon afterwards.
In the "Hunger" episode of Dilbert, Dilbert genetically engineers a new type of vegetable called the tomeato to solve the problem of world hunger. When he sets up a tomeato farm in Elbonia, the tomeato plants suck all the nutritional value out of the mud, Elbonia's primary resource. When it's discovered that tomeatos are highly explosive, the Elbonians build tomeato-based weapons of mass destruction to bully their neighbors with.
A fair few times the Urpneys managed to steal The Dreamstone were the result of one of the heroes being careless with it, usually Rufus.
In the Dr. Zitbag's Transylvania Pet Shop episode "Ants in Your Pants", Dr. Zitbag attempts to scare away his rival Professor Sherman Vermin with a swarm of termite ants. He succeeds, but at the cost of the ants proceeding to destroy every house in Transylvania.
In "The Masked Mallard", Scrooge becomes a vigilante to fight crime, but a crooked TV reporter copies the identity to commit crimes and blame the hero in his reports in order to boost ratings. Scrooge begins searching all over the city to discover the fake and expose him while clearing his own name. When he finally locates the bogus hero, right before he can swoop in Gizmoduck appears on the scene and arrests Scrooge, thus not only exposing Scrooge's identity to the public and leaving him a wanted felon but also allowing the real criminal to get away. The issue? Gizmoduck's civilian identity is Fenton Crackshell, Scrooge's accountant, and Scrooge is very much aware of this, so if he had simply told Giz the confusion wouldn't have happened.
"Where No Duck Has Gone Before": Scrooge tells Gyro to make the set "as real as [he] can." He means to make it realistic-looking, but Gyro makes it a real spaceship. The boys, Launchpad, and Courage could have avoided their hair-raising encounter with the Kronks if Mr. Gearloose wasn't so Literal-Minded.
Upon discovering that everyone has actually gone up into outer space, Scrooge destroys the computer console in front of him in frustration. Unfortunately, it happens to include the only working radio, keeping him and Gyro from contacting Courage, Launchpad, and the boys and warning them about what's happened.
In the Ed, Edd n Eddy episode "Ed Overboard", Marie and Lee kidnap Ed to pacify May, and Edd and Eddy call upon the aid of the Urban Rangers (Rolf, Johnny, and Jimmy), to help them rescue him. While Rolf was distracting the Kankers, Jimmy and Jonny proceed to help Ed down from a tree he's tied to, but Ed offers to distract the Kankers while the Urban Rangers get "help" (ignoring Johnny's protests of "We are the help!") and proceeds to blow their cover.
Exo Squad: In the episode "Heart of Mars", the Able Squad fights General Typhonus' Neosapien troops for control of an abandoned alien facility on Mars, but in doing so, cause it to self-destruct, which in turn causes the entire planet to blow into pieces. This is not as clear cut an example, since both the Terrans and the Neosapiens contributed to the destruction of the facility, but because many Neosapiens were bred in Martian tanks and see Mars as their homeland, their leader Phaeton pins this catastrophe on the Terrans (and, specifically, the Able Squad) to rally his already-losing side for a desperate last-ditch effort to defend the Neosapien-held Earth — while also secretly constructing a Doomsday Device in order to pay the Terrans in kind, by destroying their home planet, should they break through his defences.
The reason Crocker is a fairy-obsessed lunatic is Timmy's fault as well. After going back in time to find out what made Crocker like this, he discovers that his teacher had fairies himself and that a mysterious incident caused him to lose them. Timmy's attempts to prevent the incident not only end up causing the incident, but allow Crocker to get hold of Timmy's fairy tracker in the process, making him a more potent threat in the present, and when Timmy attempts to go back in time again to fix all this, he finds himself forever forbidden to time travel to that specific month.
Though Cosmo is sort of at fault here too. In the original timeline, Past Cosmo would have still blurted out the secret anyway, and in the altered timeline, Present Cosmo stupidly turning the power on the microphones led to everyone hearing Timmy mention Crocker’s secret.
"That Old Black Magic" might be the worst example, as Timmy, demanding to talk to the anti-fairies, ends up wishing them free, nearly destroying the Earth as a result.
In "Christmas Every Day", where the eponymous wish is undone after much trouble, there's a new rule stating a Godkid can not wish it was Christmas every day. Cosmo is impressed and lampshades that you have to really screw up for them to instate an entirely new rule.
During the two-part episode "Stewie Kills Lois", Brian tells Stewie that he's (in so many words) all talk which basically caused the events of that episode up to and including... Stewie actually going through with his plan to kill Lois, Stewie dumping evidence (since Brian was snooping around) which later led to Peter getting arrested for murder, Cleveland's death, and Stewie's rise to power, before being stopped by Peter. Averted the moment after, as it's revealed to be one simulation run by Stewie of how an actual murder attempt would play out.
In "Back to the Pilot", Stewie and Brian go back to the first episode via Stewie's time machine to help Brian remember where he lost a tennis ball. Once they return back to the present time, Brian reveals that he told his former self about the September 11 attacks ahead of time, allowing him to stop them from ever happening, much to the chagrin of Stewie, who warned Brian not to alter the past. While watching the local news, it is also discovered that former President George W. Bush, who has lost the 2004 election, has returned to Texas and seceded from the United States, along with the rest of the southern U.S., resulting in a second Civil War. Dismayed, Stewie questions Brian for his actions. Brian insists that things will still be better five years in the future. To make sure, Stewie and Brian then travel to the future, in which the characters appear as a series of three-dimensional objects. They then look at the outside world, only to find utter chaos and destruction. Upon research, Stewie learns that the civil war has led to nuclear attacks all across the eastern United States, resulting in seventeen million deaths (as opposed to the three thousand lives lost in 9/11).
In "New Kidney in Town", Peter becomes addicted to Red Bull, annoying, in particular, Lois, who attempts to rectify the situation by disposing of the drink behind his back. Peter then ends up drinking kerosene.
In "The Sting", he attempts a Heroic Sacrifice to protect Leela from a giant bee but only causes the bee to sting both of them, pushing the stinger through both his abdomen and hers. His body being in the way lodges the tip of the stinger in Leela's stomach, which injects her with all the venom, and put her into a very deep coma as a result. Had he not tried, she would have just had the same injury he had and been patched up the same way.
Gets everyone in the universe kicked out of the Fluffy Cloud Heaven in "The Beast with a Billion Backs" by disobeying Yivo's decree not to contact the other dimension. He sends a letter to Bender, who responds by staging a robotic invasion of Yivo.
Leela isn't immune to this either. In "Assie Come Home" she's sent to deliver a crate of guns to one of a planet's two rival gangs and decides to get Bender to bend all the barrels, rendering them useless. Turns out the guns were a peace offering from one gang to another to help them fend off the planet's resident Giant Spiders, and when the sabotage is revealed, all the gang members present shoot each other dead.
Leela: Look, let's just agree that gang violence is an important issue with no easy answers.
In Gargoyles, the heroes found a woman who had been mutated into a gargoyle-like creature by Doctor Sevarius, who was working for Xanatos. The heroes bust into the lab to rescue her, not knowing that Xanatos had forced Sevarius to create an antidote, and he was on the verge of administering it. In the ensuing fight, the vial containing the antidote is smashed, they drag the mutated woman out of a hole in the roof, and throw Doctor Sevarius into his own vat of electric eels, which means that any knowledge of how to create the antidote dies with him. Of course, at the end, it turns out that Doctor Sevarius is not dead, and that this was a Batman Gambit, except for a few minor details which Xanatos benefited from.
Later Xanatos claims that the 'antidote' was a fake. When the pseudo-gargoyles again try to make Sevarius create an antidote, it is a completely different color -and there was no reason to trust him that time, either.
Xanatos himself becomes subject to this in the episode Double Jeopardy, after attempting to create a clone of Goliath named Thailog. Thailog was exposed to an education program designed to imbue him with Xanatos' own philosophy of life. Naturally, the end result is a duplicitous and egotistical schemer prone to Xanatos Gambits - and a hatred of Xanatos and gargoyles.
In the episode "Dipper and Mabel vs. the Future," the titular twins' Great Uncle Ford talks Dipper into becoming his apprentice, which would require Dipper to stay in Gravity Falls after the summer is over. His sister Mabel, freaking out already over how terrible everyone older than her says growing up is, tries to freeze Gravity Falls in time forever with Blendin Blandin's help so she won't have to grow up and Dipper can't leave her. The only problem is, Blendin was possessed by Bill, who used Mabel's desperation to get the rift and startthe Apocalypse.
Happens again in "Weirdmageddon" when the survivors attempt to form the Zodiac Wheel in the hopes of finally stopping Bill. Unfortunately, both Stan and Ford can't stop bickering even in the middle of an apocalypse, the former accusing the latter of their current predicament and refusing to take his place on the Wheel until Ford thanks him for rescuing him from the portal, while the latter passive-aggressively snipes at Stan by correcting his grammar like a know-it-all. The whole thing ends with the Wheel being disrupted, not only ruining a chance to stop Bill once and for all, but also endangering Dipper and Mabel's life in the process. As a result of this, both Stan and Ford realize what their grudge has done and decide to finally bury the hatchet to stop Bill.
Adam at the end of episode 3 accidentally kicks a part of the spaceship that causes it to malfunction and crash in the ground. If it weren't for the fact Mira breathes underwater, he'd also have caused her death, which Kai and Adam think really happened before she shows up again.
Attempting to do a good thing, Kai releases the other team from their ice prison, against Adam's suggestions. The other team takes the moment right after they are released to finally betray the main group and take the ishibo to the Last ironwood Tree.
In Jackie Chan Adventures, the defeat of the first season's Big Bad allows the antagonists of the second season back into the world. And he was much weaker than they were. At the beginning of the third season, Jackie himself destroys the talismans while trying to keep them away from two opposing forces, the very act of doing this causes the arc for that season.
In fact, this is how EVERY season starts. Find an ancient shield with a Talisman in it? Start the great Talisman hunt. Blow up Shendu? He became a spirit that released his seven siblings. Blow up the Talismans to stop Daolon Wong? Re-start the great Talisman hunt. Remove Wong's magic? Trying to reclaim it he unleashed the Oni. Those eight Demon Sorcerers you beat? The weapons used to defeat them are now charged with demonic energy Drago is absorbing to increase his own strength. If there had been a 6th season, it probably would have explored the consequences of sealing the Oni away.
Arguably, the first Talisman hunt had already started since the villains were already hunting them. Jackie had just beaten them to the Rooster Talisman. Also, re-starting the Talisman hunt was better than letting Wong get them all.
The end of the fourth season does this as well, after spending the whole season collecting the Oni Masks Chan and Co. find the last mask and are surprised that they encounter no resistance in retrieving it and that none of the bad guys show up to wear it. But when they bring it back to the other eight Oni Masks it turns out that re-uniting them revives all the Oni Generals in their original forms with bodies of their own, something that Tarakudo was counting on.
Who do the Major League Baseball official scorekeepers credit with the "Nice Job Breaking It, Hero" on that one? At first, it seems obvious that Supes, Wonder Woman, and Hawkgirl get it but if Dr. Fate had simply said, "Wait, I can explain!" the whole episode probably wouldn't have happened. Stupid grey areas.
They both take credit, Fate for his silence and the league for jumping to conclusions. Given their past history, Superman and Dr. Fate really should've been more trusting of each other.
Though in all fairness, Fate did say once he started the spell, he was not able to stop it, which explains why he didn't take the time to stop chanting and explain everything which would have likely, took too much time.
Though Aquaman and Fate's wife had no such restriction. (Grundy had other concerns.)
Justice League has a few other moments. There's the fight between Superman and Captain Marvel in "Clash" which destroys Lex's shiny new city, though Lex was expecting Superman to jump to conclusions. The city being destroyed was a bonus. Then there's the episode where their Kill Sat gets jacked by Luthor, fired at a populated city, and they get blamed for it. Not exactly their fault, but they did make the thing without consulting anyone.
And again in "Eclipsed" when Hawkgirl, after seeing a magic crystal capable of possessing anyone who touches it finally leave a human host, decides to smash said crystal into a million pieces. Flash dodges the pieces. The rest of the league doesn't.
Ron Stoppable is directly responsible for Señor Senior Sr. becoming a supervillain. Word of advice: Maybe don't mention to the retired billionaire looking for a new and exciting hobby to get rid of his boredom just how much his house already looks like a supervillain lair.
Monkey Fist wouldn't have realized "the infant" who beat him earlier was "The Han" he was looking for... especially if Kim hadn't tried to insult his intelligence by mentioning Hana "Han" Stoppable was Ron's sister.
Kipo and the Age of Wonderbeasts: Lio Oak panic upon realizing who might be attacking the burrow led to his rash decision to throw Kipo into the aqueduct to begin with, though in his defense, no one told him that there was a second burrow. And while he is able to leave coded messages to Kipo so she could reach the new location, it never occurred to him (or anyone else for that matter) that she could easily be followed from a distance.
The Looney Tunes Show: In "SuperRabbit", SuperRabbit (Bugs Bunny) balls up Lex Luthor (Elmer Fudd)'s power suit and bats it into outer space. Lex points out that he has just sent an unstable nuclear power source into orbit. Sure enough, the suit explodes freeing Zod and the others from the Phantom Zone.
In the Martha Speaks episode, "Too Much Martha", Martha is forced to limit her portions of Alphabet soup due to her weight gain. She tries to trick Helen into giving her more by acting like her vocabulary has dropped because of it. The family buys this as Helen gives Martha more soup, but she blew it by saying, "See, Skits? I told ya it would work!"
Coop is very good at it. In one episode of Megas XLR, he finds a planet where giant sentient robots have apparently been enslaved by a mysterious alien and helps them regain freedom. At the end of the episode it's revealed, that it was a prison planet and he just broke free to most ruthless robo-criminals in the Universe. In another episode, an attack he used against a villain was so strong that he ripped a hole in reality and almost caused the end of the Universe.
Had the series continued, Coop would have been responsible for creating the Glorft, making a Stable Time Loop that allowed Coop to find Megas in the first place. That means every bad thing that happened in the entire show was Coop's fault in one way or another.
Not to mention destroying New Jersey every episode.
The episode "Breakout" might take the cake. Coop accidentally releases a planet-busting nuclear lizard-monster named "Grrkek the Planet-Killer" from his prison cartridge thinking it was a game cartridge. Then, after beating him, Coop releases the rest of the monsters in the cartridge trying to put him back in it and breaks the cartridge to try and stop it (too late, of course). Then it turns out the laser barrage Coop beat him with only made him stronger,as well as three times bigger. And Coop's disposing of the other monsters in the ocean caused a tidal wave that beat Grrkek but flooded Jersey. Said monsters then go off to terrorize Philly, but the heroes decide to let them have it.
Mike the Knight is a TV show that THRIVES on this. Every episode, Mike breaks things in the first half and fixes them in the second. Every. Episode.
Miraculous Ladybug: Lila's involvement in the series started in "Volpina" when Ladybug calls her out about blatant lying in front of Adrien, "Chameleon" being a repeat with Ladybug's civilian identity Marinette confronting Lila over lying to their classmates. Marinette's actions earned her a dangerous enemy who nearly got her expelled from school and enlisted her bully Chloé in her schemes.
There's also whole mess in "Rogercop" is started when Plagg mistook Chloé's bracelet for a box of Camembert and sneaks into her bag to get it. He then ends with the bracelet stuck on his head when Marinette trips on Chloé's bag which leads to: a.) Chloé accusing Marinette of theft, and b.) Adrien temporarily unable to transform into Cat Noir and deal with the akumatized Officer Roger until he can find a way to remove the bracelet from Plagg's head.
The end of Monster Allergy season 1 has Zick absorbing the Monster-Saur with Magnacat and his minion Viziosed inside. Hurray! But in season 2, episode 13, Magnacat manages to absorb it while being imprisoned for a long time, making the Dom Box confused it to release him and Viziosed. Oops.
Speaking of season 1, there is one. Zick having absorbed Bristlebeard, a Dark Phantom, turned out to be a bad idea since that results in Magnacat, whom he devoured in the last episode, released.
MonsterVerse series Skull Island (2023): In the Whole Episode Flashback of Episode 7, King Kong kills the rampant chameleon creatures, but the cocky way he goes about doing so puts himself and his closest companions in grave danger and leads to him sustaining several wounds, much to his human friend the Island Girl's chagrin. Where the real Breaking part comes in is that Kong had to wash his wounds in the water of an inlet; one which unfortunately was hosting the hibernating Kraken until Kong's blood woke it up, leading the monster to massacre Kong's human charges and kill many humans and animals over the course of the series and kick off the human cast's marooning on Skull Island]].
In "Dragonshy", Rarity could have easily lured the dragon out of the cave had she not told him she would guard histreasure hoard. After the dragon kicks her out, Twilight Sparkle calls her out on it.
In the episode "Swarm of the Century", Twilight Sparkle uses her magic powers to stop a swarm of locust-like Parasprites from eating all the food in town. It works; the Parasprites stop eating food and go after everything else.
In "Over a Barrel", Pinkie Pie tries to settle a territorial dispute between settlers and the native buffalo herd by singing about sharing. The sheriff and the buffalo chief finally come to an agreement...that it was the worst performance they'd ever seen. When the buffalo are on the verge of backing down from their threat of a stampede, Pinkie Pie's ill-timed reprisal of the song triggers the chief's Berserk Button.
Rainbow Dash's advice to Apple Bloom to try as many different things as rapidly as possible in order to find her Cutie Mark got carried over to the Cutie Mark Crusaders. Perhaps if they weren't constantly changing plans they might succeed.
In the second part of the two-parter episode, Twilight's guess Discord's riddle meant the Elements Of Harmony were in the maze was wrong and because of her not thinking the riddle out more carefully, her friends were broken and Mind Raped by Discord for nothing. What's worse, Discord takes great pleasure in rubbing it in her face.
Pinkie Pie messes up things again in "Luna Eclipsed", when she keeps freaking out over Luna doing something and taking it as Nightmare Moon doing something. This wouldn't be a bad thing, but Luna's just come back from 1000 years stuck on the moon and has no idea how anything runs. The third time Pinkie Pie does this, Luna initiates a Rage Quit and declares Nightmare Night cancelled.
Twilight breaks herself in "It's About Time". A battered and broken Twilight from the future appears briefly in the library and tries (unsuccessfully) to warn her past self about some oncoming disaster to take place between now and next Tuesday morning. Present Twilight misses the important part of the message and immediately freaks out, spending all of her time trying to deduce what's about to happen and, if at all possible, prevent it... but nothing really bad actually happens. Twilight finds a spell to go back in time, and tries to warn her past self not to panic — but in failing to deliver the message, CAUSES her past self to panic.
This happens again in the season two finale "A Canterlot Wedding". Seeing the very questionable actions of Princess Cadance leads Twilight to believe that she has somehow become evil and tries to warn everypony about what she's learned. This makes her come off as rather crazy and her friends end up shunning her, including Celestia. Surprise! Cadance really isn't evil, but the Cadance that was acting like a total Bridezilla is actually an EVIL IMPOSTER... Specifically a new foe, the Changeling Queen Chrysalis, impersonating Cadance and using her love for Twilight's brother, Shining Armor, to fuel her power. By the time Twilight and the real Princess Cadance arrive to stop her, the Queen already became too strong for even Celestia to handle. She even points out that they could have stopped her sooner if they all had listened to Twilight in the first place, although as Twilight herself points out a moment later, her disguise would have fooled everypony anyway. It was only because the real Cadance was Twilight's fillyhood friend that she grew suspicious of Chrysalis!Cadance to begin with, and even then she only realized the real truth after finding the real Cadance in the Canterlot caves who informed her of the imposter plotting to take over Equestria.
Applejack: Sorry, Twi... We should have listened to you. Twilight Sparkle: It's not your fault - she fooled everypony!
Twilight Sparkle pulls practically the exact same blunder she did with the Parasprites again in "Bats!" She tries a spell that will change the vampire fruit bat's tastes so they don't like apples and it works... with the side-effect of transforming Fluttershy into a vampire fruit pony with an even bigger appetite than all the bats combined.
In "Somepony to Watch Over Me", Apple Bloom secretly runs away from home while Scootaloo and Sweetie Belle take turns posing as her in bed. While Sweetie Belle is in bed, Scootaloo hides in the closet. Applejack then says she doesn't have to look after Sweetie Belle anymore, and Scootaloo shouts out loud, "Wow! Wait until Apple Bloom hears about this!" Then the jig was up!
In "Equestria Games" the Power Nullifier to prevent cheating from the unicorns prevents them from doing anything to stop the giant ice cloud as it plummets toward the stadium.
Celestia's tradition of showing off the heroes of Equestria (notably, Princess Twilight's ascension) in the stained glass windows of the royal halls comes back to ruin her plan to keep knowledge of Twilight secret from Tirek. He probably would have found out anyway, but still...
Discord puts his own spin on most of the windows in the palace, but forgets to do that specific one, or at least hadn't gotten to it by the time Tirek noticed.
Canter Zoom gets his niece a job at the popcorn stand of Canterlot Mall's movie theater. The rather humiliating job coupled with the Daring Do movie premiere and the "Dance Magic" music video playing frequently through the mall doesn't help Juniper's mental state any, even before she gets her hands on the magic mirror.
The girls pay an accidental visit to Juniper at said popcorn stand, not knowing nor noticing her resentment and increasingly unhinged demeanor. What pushes Juniper over the edge is when Rainbow Dash reminds her that she put herself in that situation by stealing the movie props. When Juniper tries to show them what she sees in the magic mirror, the girls whisper within earshot some rather unkind things. Juniper finally snaps and wishes them all to leave her alone, the magic mirror activates and sucks them all inside.
Applejack: Two apples short of a bushel, I say.
In "Amending Fences", we find out that Twilight's Establishing Character Moment from the very first episode broke another pony altogether: Moondancer was a very shy pony who was starting to come out of her shell as she considered Twilight the closest thing to a friend. When Twilight snubbed the party invite due to her studies and was ordered to go to Ponyville by Celestia, ignoring the party and not even telling her friends goodbye, this turned Moondancer into a reclusive shut-in, focusing only on studying. It takes Twilight five years (in Real Life terms) to realize she made a big boo-boo.
And Pinkie Pie gets a hat trick. In "Rock Solid Friendship", Maud Pie and Starlight Glimmer are bonding and becoming friends, but not at Pinkie Pie's pace. Her constant pushing ends up driving Maud Pie out of Ponyville and into the Ghastly Gorge, nearly to be Eaten Alive. As well, we find out that Maud had unwittingly revealed a special type of stone to Starlight that would suppress the magic of a Cutie Mark back when she was starting up "Our Town".
In "Shadow Play", Twilight discovers that Star Swirl the Bearded and his allies, the Pillars of Equestria, had sealed themselves in limbo for over a millennium in order to contain the villainous Pony of Shadows. Eager to meet her idol, she works to find a way to rescue the legendary ponies from their fate. She ends up succeeding in this regard after her friends gather items belonging to the Pillars. Unfortunately, the Pony of Shadows is brought back right along with them.
Phineas and Ferb: In "Candace Gets Busted", Perry pushes the reverse button on Doofenshmirtz's inator, transporting Candace's party back to the Flynn-Fletcher house and getting Candace busted.
In one episode where the three girls and Professor Utonium move to a much larger city than The City of Townsville, which is reversely named the Town of Citysville, The Powerpuff Girls think it's a good idea to continue fighting crime in their new location. After stopping a couple of robbers by destroying the bridge they were trying to get away on, the mayor of Citysville complains to them. "At what point did you think it would be a good idea to blow up the Citysville Bridge? Those robbers stole $4000 in cash. But it's going to cost MILLIONS of dollars to replace the bridge!!!" This catches the girls off guard because, in every single episode prior, plenty of buildings get destroyed Godzilla-style and nobody seems to care a thing about it, as long as the monster is gone. Moral: There is a significant difference between destroying a few buildings in the process of preventing a giant monster from razing the entire town, and stopping a pair of petty crooks.
This is also a major part of the premise of The Movie, where Mojo tricks the girls into helping him build his volcano observatory and a machine that he claims will "help the town and make it a better place" but actually helps him take over.
Bubbles: This isn't making the town a better place!
In "Fallen Arches", a group of elderly super-villains known as the Ministry of Pain come out of retirement, but Blossom refuses to fight them out of a misguided sense of "respecting your elders". This only convinces everyone that the Powerpuffs are afraid of the Ministry's notorious reputation. Blossom then tries to convince the equally-elderly superhero duo of Captain Righteous and Lefty to settle their differences and come out of retirement. The episode ends with all the elderly supers hospitalized, and a news reporter talking about "None of this would have happened if the Powerpuff Girls had just saved the day!"
There's also the episode where the girls have a race and go so fast, they warp into a Bad Future where the Him took over because the girls weren't around (due to their race). Ultimately the girls manage to get back to the present, averting such a future from occurring.
In the episode, "Ice Sore", Blossom tries to stop crooks with her ice breath, but it actually made them get away even faster.
Before Princess' Face–Heel Turn, this trope occurred at her first attempt at being heroic. Due to having no superhero experience, she almost gets herself killed while causing the robbers to get away.
In "Power Lunch", the Girls accidentally give the Gangreen Gang super-powers when the radiation from the Powerpuffs' heat vision reacts with the massive amounts of junk food the Gangreen Gang ate after ransacking a convenience store.
When Mac Antfee returns, he's a motivational speaker and becomes nice. Randy believes it's an act and reminds Mac of the ninja mask with all its cool weapons, powers, and the Nomicon, causing Mac to turn back to his old self while practically exposing himself as the Ninja. Randy also accuses the Nomicon of doing this for not mind-wiping Mac when it fired him as the Ninja.
In the Ready Jet Go! episode "Moon Circus”, Jet blabs out loud that they're planning a surprise party for Carrot, unaware that Carrot is listening. Carrot also gets a moment of this when he says out loud that he knows they're planning a surprise party for him.
After being caught in a snowstorm in Christmas Season, the Ghostbusters find themselves in an old-fashioned place, where they see an old man being tormented by 3 ghosts. They charge in and capture the ghosts, apparently saving the old man. Upon returning home, the Ghostbusters find the people around them have become very selfish and that they all hate Christmas. They soon realize that they actually had just gone back in time; that the old man was actually THE Ebenezer Scrooge; that the ghosts they captured are THE Ghosts of Christmas Past, Present, and Future; and that they have prevented the events of A Christmas Carol from taking place, leaving an unrepentant Scrooge to provide a different written account, replacing A Christmas Carol, with an opposite message of selfishness to the world.
In "Bustman's Holiday", Ray inherits a castle in Scotland and the Ghostbusters go over to check it out. They stumble upon what the castle's caretaker calls a "keystone ghost" and capture it, but soon more ghosts start popping up. The caretaker reminds them that it was a keystone ghost, and that by catching it, they released the other ghosts in the castle. All 360-some of them.
On Regular Show this happens on a pretty regular basis. For instance, the second episode features Mordecai and Rigby desperate to stop a rampaging video game monster, all because Rigby ignored the warning not to active the video game booth.
Rather than take Citracet to jail for destroying the world again, The Ripping Friends decide to hammer him into the center of the Earth. Once there, he gains super-powered farting powers and returns as "Stinky Butt the Foul": the most powerful villain they've ever faced who easily takes down all four of them at once. They only won because Stinky Butt ran out of gas and had to retreat, giving them enough time to devise a plan.
In one of the Rocky and Bullwinkle serials, the duo learn that Boris is planning to rob the Frostbite Falls bank and attempt to warn the guards on site. Unfortunately, Bullwinkle's plan on warning everyone is to burst into the bank and shout "Hey everybody, it's a stick-up!", causing the guards to start shooting at him and Rocky thinking they're the criminals. And they're so distracted by Rocky and Bullwinkle that Boris is able to break into the bank without any resistance.
Samurai Jack; in the episode "The Birth of Evil", a well-meaning samurai lord (Jack's father) led his army against a pool of primordial, evil goo. Upon reaching it he fired an arrow dipped in a mystical poison into the pool to destroy it, but gave it sentience instead and created Aku.
Episode 26 of Scooby-Doo! Mystery Incorporated has Velma being blamed for the dissolution of the gang after the ep's "ghost" is unmasked to be the Mayor of Crystal Cove, a.k.a. Fred's dad—or rather, surrogate dad, as Fred's real parents are missing, and that she kept Angel Dynamite's real identity a secret from the gang.
The Secret Saturdays: Honestly, Zak's entire deal with Argost in Season 2 counts. Not only does Argost emerge the victor, but it allows Argost to access the Saturdays' computers and learn about Zak's Evil Doppelgänger Zak Monday, giving Argost the idea to summon Zak Monday and steal his antimatter Kur powers for himself.
"Simpsons Bible Stories": In a send-up of the David vs. Goliath legend, King David (Bart) fights Goliath II (Nelson) and loses, with Goliath II taking over as king. After some Training from Hell, he comes back and does a Nice Job of slaying the giant. The Breaking It part comes when he's told that Goliath II was the best king the city had ever had, building roads, libraries, and hospitals, and David is arrested for megacide. Whoops!
Arguably parodied in "You Only Move Twice", when Homer takes a job in another city working for HankScorpio. Homer turns out to actually be pretty good at his job, successfully setting up the nuclear reactor Scorpio needed for his plan to succeed, and unwittingly preventing Scorpio's plans from being derailed when "JamesBont" tries to stop him. When the family moves back to Springfield, Scorpio sends Homer a thank-you note, stating that he couldn't have taken control of the East Coast without Homer's help.
Turned up to eleven and deconstructed in The Simpsons Movie, where Homer lands Springfield in more trouble than it's ever been, completely pisses off his family, and spends the rest of the film facing the consequences of his actions and trying to fix everything. Although at least he makes it all right again by the end and also sums up modern heroism surprisingly well:
"Risking my life to save people I hate for reasons I don't quite understand!"
Homer subverts this trope in "Boy-Scoutz 'n the Hood" when Bart joins the Junior Campers (an Expy for the Boy Scouts) and has to take Homer along on a father-son river rafting trip. They're paired with Ned Flanders and his son, and Homer makes things go From Bad to Worse. Homer loses their map and gets them separated from all the other father-son pairs, and they end up drifting out to sea. They quickly run out of food and water because of Homer's Jerkass behavior, and he makes their raft spring a leak. However, Homer ultimately saves them when he realizes that there's a Krusty Burger on a nearby offshore oil rig, and his amazing sense of smell leads them to it. They get food and a ride back to Springfield, and Bart congratulates his father. It also turns out that Homer actually did Bart and the Flanderses a favour by getting them separated from the other rafts. We see what the other father-son teams go through, and it turns out to be a rather unpleasant experience.
Played somewhat straight at one point, where it's implied that he stole one of the fathers' swiss army knives, which resulted in him being forced to retreat with the kids after discovering it was gone.
"Burns, Baby Burns" had Marge telling Homer to go out of the house and tell Mr. Burns about the kidnapping being a hoax along with Burns' son. Her advice only resulted in Homer getting into even more hot water than before when he takes Burns' son out in broad daylight, something that Homer even attempted to warn Marge about why he shouldn't do that immediately.
Kent Brockman: Appearing in broad daylight with police everywhere, ladies and gentlemen, there's only one word for that... idiocy.
*Marge groans embarrassed*
In Sonic Underground, the Oracle tells the Queen the prophecy of Robotnik's rise and eventual fall thanks to her and her children. He then proceeds to use this knowledge as Unsportsmanlike Gloating against Robotnik, thus alerting him to the threat's existence in the first place and motivating him to actively seek them out.
In "Spontaneous Combustion", people have been spontaneously combusting. Scientist Randy Marsh discovers that this was caused by people holding in their farts, so he tells people not to hold it in. Unfortunately, the constant release of flatulence begins to cause global warming.
"Pandemic 2: The Startening": Who knew that Peruvian Pan Flute Bands were the only thing holding back the hordes of Mutant Guinea Creatures intent on destroying the Earth? The Secretary of Homeland Security, that's who.
"Smug Alert": And who would have known that hybrid vehicles while emitting less smog, they tend to make the people who drive them emit a more obnoxious gas called smug?
The "Imaginationland" episodes begin with Cartman and his friends trying to capture a leprechaun (with Kyle agreeing to suck his balls if Cartman proves his claims, as per the Black Comedy nature of the show), but once they capture it, the leprechaun proclaims, "I was sent to warn of a terrorist attack, but you boys have made me late. Now the terrorists will prevail! The end is near!"
In "Woodland Critter Christmas", Stan is sent by the critters to kill a mountain lion who has been terrorizing them for years. He succeeds in the deed and later finds out she had cubs and would die without her. To make matters worse, the critters are evil and wish to resurrect The Antichrist, which the mountain lion was trying to prevent. Then again, this is a story by Cartman.
Star Wars Rebels: In "Through Imperial Eyes", the rebels stage an operation to rescue Kallus, The Mole, because they think one of his Fulcrum transmissions may have been tracked by the Empire. Ultimately, this only makes things worse, because due to their interference, Ezra accidentally leaves behind a piece of evidence, a helmet painted with Sabine's distinctive artwork, which allows Thrawn to deduce that Kallus is the traitor in the Imperial ranks. In addition, despite the rebels' attempts to delete Atollon from the list of planets Thrawn suspected of potentially hosting the rebel base, it turns out he didn't need the list because he could use Kallus to find it.
Star Wars Resistance: In "Bibo", Neeku immediately adopts the titular creature as a pet after finding it in the wreckage of a salvaged starship. This draws in Bibo's much larger mother, who attacks the Colossus due to wanting her child back.
Steven Universe: Has this on numerous occasions, but one moment manages to mix this with Chekhov's Gag. When Steven talks to Peridot for the first time in season one's "Marble Madness", he lists off the names of some of his friends and family in an attempt to make friendly conversation with an alien bent on killing him. This comes back at the end of season four, where the list is used to locate and kidnap said people for Homeworld's People Zoo and eventually leads to Homeworld realizing that Steven is his mother, which in turn renews the Gem Empire's interest in the planet Earth.
Stroker and Hoop pretty much abide by this trope, no sooner than they think they've fixed things. It only proceeds to get worse due to their blundering.
Supa Strikas: Invincible United used a form of hypnotic therapy to turn Dooma from a violent thug who did nothing but injure other players to a highly skilled captain who turned the team around from being way out of the running to being in the Super League finals. The Supa Strikas finding a way to sabotage his therapy and turn him back into a thug (and thus giving away crucial penalties) could be seen as a type of Nice Job Breaking It, Hero
This frequently happened in Superfriends, especially during the Challenge of the Superfriends era with Lex Luthor and the Legion of Doom. A common plotline was for the Legion to con the Justice League into doing their dirty work, retrieving whatever phlebotinum the plot requires for the villains to threaten the world. In one notable episode, Luthor developed the technology to control solar flares and sent an enormous solar flare to incinerate the Hall of Justice. The Justice League triggered the HOJ's global force field to prevent the flare from impacting the Earth. The flare instead bonded with the forcefield, spreading across the upper atmosphere and scorching the Earth, killing all life. A trio of Sufficiently Advanced Aliens chose to reverse the flow of time to avoid this catastrophe, and then moved the moon to eclipse the sun to block the flare. There were no signs of destructive tides as a result, oddly enough.
At the end of the episode "Trust", Robin gives a communicator to Hot Spot. Except it wasn't Hot Spot but the Voluntary Shapeshifting villain he fought the whole time. This allowed the Brotherhood of Evil to track all of the Titans' allies. While Robin didn't intentionally give the item to the villain, he was handed the Idiot Ball so he didn't suspect the possibility.
Occurred at least one time in the first TMNT series involving Donatello's inventions such as when Donatello created a clone of himself to do all the handyman's work. Except his clone doesn't follow orders and a glitch made him stronger and smarter than him. The clone soon worked for the mob boss Pinky McFingers to create millions of cloned rats to attack the city. It's up for the Turtles to fix this.
Occurs twice in the second Teenage Mutant Ninja Turtles series: first when the turtles "kill" the Shredder in the "Return to New York" which sparks a gang war between different factions trying to fill the power vacuum; the second when they steal the Heart of Tengu from The Foot to trade for a cure for the mutated Donatello. Unbeknownst to them, the Heart was the only thing binding a quintet of elemental mystics to the Foot's will; with its theft and subsequent destruction, the mystics were free to resurrect their master, the original (and demonic) Shredder.
Happens about once per episode in Rise of the Teenage Mutant Ninja Turtles with the best example being the series premiere where the Turtles battle with Baron Draxum destroyed the villain's fortress but wound up releasing a plague of Oozesquitoes upon New York who promptly began creating a horde of mutants. To be fair to them, Draxum was likely going to unleash them soon anyway (since the mutation worked), and at least this way, they knew what was going to happen and could fight back instead of being completely unprepared.
Zudo of The Telebugs only became evil because Grumble the cat interfered during his creation.
Late in ThunderCats (1985), Mumm-Ra fails for the last time, so the Ancient Spirits of Evil banish him to a pocket dimension as punishment. The heroes decide to celebrate by blowing up Mumm-Ra's pyramid. This pisses the Ancient Spirits of Evil off so much that they bring Mumm-Ra back and give him another chance.
In another episode Cameron points out that the fights were rigged, resulting in Mike/Mal facing Izzy in a spider suit (Cameron's fear), and Cameron facing mutant gophers (Mike's fear) instead of the other way around.
In one episode of Totally Spies!, the girls visit a small town in Italy, only to find it deserted, as a duel between two pizza-wielding brothers has chased everyone else out, after briefly fighting them both, the girls agree to sample each pizza and decide who is the better chef for them. The girls decide they are equally good and advise them to set aside their differences. The brothers tearfully reconcile and agree to work together, the girls happily leave, assuming they mean to set up a joint restaurant. It turns out, they decide to pull a Villain Team-Up and destroy all other pizza chefs. So, yeah...
In Transformers: Prime, Bulkhead's crashing of the Nemesis in an attempt to escape it inadvertently prompted Megatron to relapse back into his Dark Energon habit.
Transformers: Animated has a Whack A Mole episode in which the wrong Autobot was arrested. Wasp goes on to become insane and vengeful in solitary confinement until he finally breaks out to get revenge on Bumblebee. Longarm, who's really the Decepticon Shockwave, goes on to become a Prime and the head of Autobot intelligence. Well, slag.
There's also the first Season Finale, where Prime decides the AllSpark key isn't safe with Sari and has Ratchet hold onto it... who then loses it a couple scenes later as he was alone when trying to defend it.
At the very end of Beast Wars, Optimus and the Maximals stop Megatron from destroying the Ark and the Autobots on it, preventing a time storm that would destroy the Maximals and made the Predacons dominant. They then tie Megatron to the bottom of the shuttle and bring him back to Cybertron to stand trial. All seems well until Megatron manages to break free from his shackles in a time warp, landing on Cybertron long before the Maximals, becoming a dictator, and extracting the sparks of every Cybertronian on the planet, leading to the events in Beast Machines.
From the villainous perspective: according to the Beast Wars: Transmetals game, had Megatron's plan to kill Optimus Prime succeeded, there would have been no Autobot Matrix of Leadership to stop Unicron from destroying Cybertron, dooming both races.
Silverbolt pulls this off twice in the second season. In "Bad Spark", he unthinkingly detonates the energon crystals on which Protoform X's stasis pod has crashed, heavily damaging himself and his comrades Cheetor and Optimus Primal as well as releasing the monster that would become Rampage — in fact, it's possible that the energon storm he created might have actually jolted Rampage back to life in the first place. And then, in "The Agenda", he helps Blackarachnia open the blasted-shut tunnel leading to the Ark, thus giving Megatron direct access to the Autobots and Decepticons contained within and giving him the opportunity to attempt his "destroy Optimus Prime to change reality" gambit. Love Makes You Dumb indeed.
Optimus Prime has the Allspark thrown into a Space Bridge set to random coordinates in order to keep it out of Decepticon hands in Transformers: Cyberverse. While this thwarts Megatron's plan to create an endless army of Decepticon soldiers, this course of action has an unforeseen side-effect. Cybertron itself begins to sicken and die.
Tangled: The Series: In "Queen For a Day", allowing Rapunzel to take over Corona, who has no knowledge of ruling in the first place, causes Corona to fall to its weakest state leaving it vulnerable enough for Zhan Tiri's blizzard curse to strike.
After Jim and Strickler from Trollhunters manage to capture Angor Rot in the same magical net he trapped Jim in a previous episode, Barbara accidentally frees him after hitting him with a shovel, thinking that she was helping. She apologizes for this later on.
The title character of Tutenstein pretty much screws up in every episode.
In Season 4, Bloom, Roxy, and their friends finally manage to reach the forgotten island of Tir Nan Og, where all of the Earth's fairies are imprisoned, and set them free. Yay, awesome! Except that Morgana, the Queen of the Earth Fairies, has decided that Humans Are the Real Monsters and that they deserve to be destroyed for ruining the Earth and for not believing in magic anymore. And all the other fairies agree with her, and have now declared the Winx girls to be their enemies for not going along. Oops.
At the end of The Secret of the Lost Kingdom, Bloom and Sky destroy the Obsidian Dimension in order to destroy Mandragora and the spirits of the Three Ancestral Witches and free the King and Queen of Domino (as well as restore said planet to normal). However unknown to our heroines this also frees the spirits of the Three Ancestral Witches to seek out and team up with their descendants the Trix.
Wunschpunsch: In "Nice Wizards", Bubonic and Tyrannia tried to cast a spell to make their supervisor a nice man who wouldn't bother if they did their work or not. Instead of simply allowing them to do so, their pets interfered with the spell and Bubonic and Tyrannia were turned nice instead. The problem with this is that they're so clumsy their attempts to use magic to do good deeds end up causing disaster they're oblivious to. The pets decided to break the spell.
Xavier: Renegade Angel is pretty much the poster boy for this trope. As Xavier's attempts to "help" people invariably lead to the deaths of hundreds.
This happens multiple times per season in Xiaolin Showdown. The biggest screw-ups are caused by Omi in the finales of seasons 2 (used only 1 half of the Yin-Yang Yoyos, causing him to turn evil and join Chase Young) and 3 (froze himself to travel to the future, only to find that Jack Spicer became an Evil Overlord and made it a Bad Future because he wasn't there to help stop Jack).
The episode in season 2 when Kimiko breaks the Mosiac Scale, releasing Sibini and resulting in Clay being possessed by him, giving him the chance to possibly get the Monarch Wings, which, when combined with the Mosiac Scale, would give him ultimate power. It's funny because it also literally broke, so it fits into this trope twice as well.
When the Heart of Jong is recovered by the bad guys, the Master reveals that when the heart combined is with certain Shen Gong Wu, very bad things will happen. End of the world bad. Fortunately, most of the relevant items are in the monks' possession. The Master gives all the Shen Gong Wu to the four kids and tells them to hide; they leave, and when the bad guys question him later, the Master makes it clear he can't say where they've gone because he doesn't know. (This is important.) Raimundo gets it in his head to take some Shen Gong Wu and go help the Master; it turns out ghostly Wuya can read minds, and finds out where the kids are -and the rest of the Shen Gong Wu needed to revive the Mala Mala Jong -an evil powerful warrior.
In X-Men: Evolution, Magneto destroys a Giant Spider that he thinks is the second key to releasing Apocalypse. Turns out the spider was actually a guardian preventing Apocalypse from escaping. Destroying it opened the second door. Oops.