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. Way to go, Jade... 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.
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 an episode of The Powerpuff Girls, 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 supervillains 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 how the whole farce could have been avoided if only the Powerpuff Girls had done something. Nice going, Blossom!
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.
The Simpsons has a send-up of the David vs. Goliath legend, in which 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 the episode "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 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, in his defense, not only does he make it all right again, he 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 Scouts 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*
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.
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.
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.
In an episode of South Park, 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.
Speaking of South Park... 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.
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.
Sheila in The Movie when she unwittingly allows Satan and Saddam to rise by killing Terrence and Phillip. Though in her case, the term "hero" is used loosely.
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.
Stroker and Hoop pretty much abide by this trope, no sooner then they think they've fixed things. It only proceeds to get worse due to their blundering.
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.
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.)
The "Gaang" in general could've seen the Kyoshi Warriors and made sure that it was them. Otherwise, they would've realized that it was Azula's group instead and they wouldn't have learned about the invasion plan.
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.
In the The Legend of Korra episode Beginnings, 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 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.
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.
In Wolverine and the X-Men, Wolverine goes and gets himself captured by going in alone. Trask analyzes his powers, which leads to future beastlike Sentinels with Wolverine Claws and a Healing Factor. Especially surprising (and satisfying) because otherwise the series takes Wolverine's Canon Sue status Up to Eleven. The only one typically allowed to do anything right (except Nightcrawler in his solo episodes) is the one who had a big hand in screwing up the future due to his own bullheadedness.
In Brother Bear, the central point of the story is that Kenai pursues and kills a bear he blames for his brother's death. Later on, he realizes to his horror that was Koda's mother.
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 other episode an attack he used against a villain was so strong, that he ripped 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.
Don't forget destroying New Jersey every episode.
In Season 4 of Winx Club, 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 any more. 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 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 Tree Ancestral Witches to seek out and team up with their descendants the Trix.
This also happened in The Real Ghostbusters. 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 went 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.
In the Direct To DVD film Tinkerbell, Tink's reckless actions ruin all of the fairies' preparations for spring, threatening to cause a chain reaction that disrupts all the seasons, which would result in an ecological catastrophe. Only her brilliant inventions get the fairies back on schedule.
Ben 10: Alien Force has an example of this in "Time Heals". Gwen successfully goes back in time and prevents Kevin's monstrous mutation ...only for her to find out that her meddling in time has caused a Bad Future were Hex rules the Earth, Kevin's 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.
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).
Let's not forget 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.
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 with 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.
Used as a Secret Test of Character in an episode of Adventure Time. 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 season 4 Finale. 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 in the mirror world seen in King Worm Wonder how they'll fix that!
Late in Thunder Cats', 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.
Ron Stoppable, of Kim Possible, is directly responsible for Señor Senior Sr. becoming a super villain, after pointing out how easy it would be for him to turn his vacation home into an evil fortress.
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.
In Gargoyles, the heroes found a woman who had been mutated into a gargoyle-like creature by Doctor Tim Curry, who was working for Xanatos. The heroes bust into the lab to rescue her, not knowing that Xanatos had forced Doctor Tim Curry 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 Tim Curry 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 Tim Curry 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 Curry create an antidote, it is a completely different color -and there was no reason to trust him that time, either.
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 territory 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 Cadence leads Twilight to believe that she has somehow become evil and tries to warn everypony about what she's learn. This makes her come off as rather crazy and her friends end up shunning her, including Celestia. Surprise! Cadence IS evil, but it's actually the Changeling Queen impersonating her, using her love for Twilight's brother, Shining Armor, to fuel her power. By the time Twilight and the real Princess Cadence 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 Cadence was Twilight's fillyhood friend that she grew suspicious of Chrysalis!Cadence to begin with, and even then she only realized the truth after the real Cadence informed her that the latter was a fake.
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 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 the 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.
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.
At the end of the Teen Titans 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.
Beast Boy harshly telling Terra that they weren't friends after finding out she had betrayed them to Slade despite the latter's regret. It caused Terra to be pushed off the edge and have a Face–Heel Turn.
Even earlier than that was Robin accidentally revealing Terra's lack of control over her powers in front of everyone, making Terra think Beast Boy had lied (he didn't), which led to her running to Slade in the first place.
During the two part Family Guy 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 Peter to getting arrested for murder, Cleveland's death, and Stewie's rise to power, before being stopped by Peter. Jossed the moment after, as it's revealed to be one simulation run by Stewie of how an actual murder attempt would play out]].
In the episode "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.
The title character of Tutenstein pretty much screws up in every episode.
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.
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.
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.
Cyberchase: In the very first episode, 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.
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.
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.
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.
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.
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.
The Brave Little Toaster: After learning that the abandoned house they were left in is now for sale, five appliances head out to reunite with their "master" and go through numerous perils trying to find him. However, their now-grown up "master" is heading to college, and he returns back to his old home to pick up some of the leftover stuff to use at his dorm (namely, them) roughly about two days after they left. They could have avoided all the trouble they went through if they had just listened to Kirby the vacuum cleaner earlier and stayed at the house.
DuckTales: 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. The real Masked Mallard 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 the hero, 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.
This is partially Scrooge's fault. Gizmoduck's civilian identity is Fenton Crackshell, Scrooge's accountant, and Scrooge is very much aware of this. Yet he never attempted to bring Giz into the loop. (Granted, Fenton is the resident Butt Monkey...)
Of course, Fenton likely wouldn't be much help anyway. After catching him, Scrooge reminded him that he was present at the same time as the faux Masked Mallard while he was committing one of his robberies and thus couldn't be the crook... and Fenton still couldn't put two and two together.
In the episode "Where No Duck Has Gone Before", 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.
The Jetlag Productions version of Sleeping Beauty adds this twist to the story. After princess Felicity pricks her finger and falls under the evil fairy Odelia's curse, Odelia actually thanks Primrose, the fairy who altered her death curse, for setting it up. Odelia explains that she just wanted to kill Felicity, but Primrose has made it so that Felicity will remain unchanged for a hundred years, and when she wakes up, everyone she ever knew and loved will have been long dead. Conversely, Felicity's parents are left to moan that they're going to die knowing their daughter will be all alone when she wakes up, as they grow older she won't age a day.
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 on Rocky and Bullwinkle that Boris is able to break into the bank without any resistance.
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 allow them to do so, their pets interfere 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.
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 hair dryers 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.
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 the threat's existence in the first place and motivating him to actively seeking them out.
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.
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 another episode Cameron points out that the fights were rigged, resulting with Mike facing Cameron's fear, and Cameron facing Mike's fear instead of the other way around.
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.
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.
On Regular Show this happens on a pretty regularbasis. For instance, the second episode features Mordecai and Rigby desperate to stop a rampaging video game monster, all because they ignored the warning not to active the video game booth.
In the 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.
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.
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 Gravity Falls 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 start the Apocalyse.
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".
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.