"Lee" and Smellerbee force Jet into therapy in The Stalking Zuko Series in hopes of curing his obsession with "Lee". Getting therapy ends up helping Jet realize that "Lee" is probably not a firebender...But his uncle Mushi is since Mushi is constantly making gallons of tea without firewood and goes naked in their poorly insulated apartment in the middle of early spring.
The Northern Water Tribe in Reluctant Hero outright abucts the new Avatar when he's found on a Fire Nation ship, forces him to assimilate while he's obviously struggling with his status and the expectations coming with it, and considers to wipe his memory clean in order to make him a better student dedicated to save the world. The teen ends up by freaking out and running away.
Riding The Dragon has Cecilia Kurita oppose her brother's coup only to leave herself in the position where she has no choice to claim the throne she never wanted in the first place, since anyone else who took the throne would have to kill her as a rival.
Chosen To Destroy, written as a Take That! at Buffy the Vampire Slayer and Angel, which has the First Evil show up after the Season 7 Buffy finale, basically to gloat over how it's going to open all the world's Hellmouths and kill all the Slayers in the world, and to rub it in that it's all Buffy's fault - while some of it was the First's manipulations, it makes it very clear that the main reason it's going to win now is because of Buffy's ego and stupidity (and Angel's to a lesser extent).
The Trans-Galactic Republic tries to "save" their neighbors from the Reapers with their powerful starships...and inadvertently stokes interspecies tensions, denies the Magnetic Hero known in canon to have pulled the feuding galaxy together, and puts their heads in the sand when the Tyrant Takes the Helm despite having the ability to put a stop to it through their superior firepower.
The Vault Hunters clear out a Hyperion spaceport with a huge, Siren-powered rocket launcher...except the detonation also destroys their only way offworld (until Lilith decides to use her powers).
In Origins, Samantha Shepard needs to hack a terminal, but sends Legion (her best hacker) away to solve another problem of tailing a thief before realizing how stupid this decision ends up being. She then proceeds to almost get sniped until Garrus saves the day.
For a given definition, Aria manages to seize enough power to keep her station by hijacking Trans-Galactic Republic ships...except now those ships aren't used properly and the Alien Invasion gets out of hand when their original commanders would have just blown it all up.
Samantha Shepard thinks she can help Cortana with a different computer system, but all this does is start playing AI Is A Crap Shoot straight. Subverted later in that it actually ends up helping in the long run.
On a much larger scale, the Neglectful Precursors who thought their convoluted plans would keep the universe from imploding or being overrun by an Alien Invasion they'd fought off eons ago. Instead, they blunder right into this trope by influencing, if not directly causing, the things they worried about (rampaging lifeforms, alien invasions, wars among the lesser races, abuse of Psychic Powers). All because of a religious spat...
Underestimates the Karate Club guys and gets in a position where she has to use a chi burst to defeat them in public, while Gauron is spying on Kaname nearby.
Naiki's fear of flying combined with a terrorist takeover of the plane she's on causes her to go on a fear-crazed berserk as soon as the plane lands, which leaves her captured and used as a hostage to insure Deimosu's surrender.
Goes back to help Kurz's fight against the Venom Chodarl, splitting the escape party and causing Eija, Kaname, Deimosu and Sousuke to hesitate long enough figuring out where she went for the Musk to attack and force Eija, Kaname and Sousuke to miss the escape.
Tries and fails to rescue and protect Tessa from the Behemoth terrorists.
Uses a black magic spell that causes her to touch Tessa's soul, starting the mind merge process and also indebting her to a Demon that makes Mara queasy. Twice.
In backstory, Naiki's intense fear of flying is due to an...interesting 10 meter trip through a hangar that occurred when she tried to operate a helicopter as a young girl.
In the fight against the Behemoth, uses her summoned tentacles to throw Mao into a pile of debris, almost effectively taking her out of the fight.
In the Martial Arts Takeout Race, loses control of her newly made Torrent of Debris spell.
Before getting one of Persephone's peaches, she spends a lot of the fight against Poseidon being rescued by the older, human martial artists.
Niamy of Fuck The Jesus Beam decides to deal with the Bombignat threat by releasing O.B.A.M.A. from his prison, despite him having started 95 World Wars as president and, as it is later revealed, being the source of the Bombignats. Upon his release, O.B.A.M.A. says that he will reclaim his domain, and takes control of the Bombignats.
While not much of a hero, Vash of Christian Humber Reloadednote A crossover between too many works to list here kills a dragon named Le-Hung Doe as part of atoning for killing another dragon, but learns that by killing him, he has started a Ying-Yang War, in which he must fight demons without weapons.
The Prayer Warriors have killed many people in their fights against the Greek Gods, Hogwarts and Communist Russia, but it's implied in The Titans Strike Back that their infiltration and destruction of Hogwarts resulted in many Christian deaths, and now that the school has been re-established, they're going to deal with it differently this time.
The God Empress of Ponykind: Celestia kills King Sombra. This allows him to ascend and become a Daemon Prince and pull the Crystal Kingdom into the Warp.
Horseshoes and Hand Grenades: Ryusei's attempts to save his friend from the coma by killing Gentaro only made Ophiuchus resurrect Gentaro as a Serpent-bearer with a vengeance to kill his former friends. Moreover, Jiro died anyway, and was resurrected by Gentaro to serve him and kill Ryusei. And that's not talking about how this derailed Gamou's plots for the Day of Awakening and how his benefactor Tachibana—Virgo Zodiarts—has to start injecting Cosmic energy into innocent kids to make up for everything. Was killing Gentaro worth all of this in the end, Ryusei?
In The Swarm of War, after the Kingdom learns about the Zerg and misidentifies them as a Chaos corruption, a decision is made to bring out the hidden superweapon: hibernating Space Marines. Unfortunately, nine of them are corrupted by Chaos.
In The Boy With The Magic Notebook, Maxwell tries to summon a healing rainstorm to heal people after Shatterbird's attack on Brockton Bay. But while it did heal all their wounds, it still left the glass pieces inside them.
In End of the Line, Sailor Moon created a magic barrier to protect her hometown from an apocalypse. As Xander furiously explains to her and Luna before he kills them, that very barrier is what caused the apocalypse by pulling magic out of everything in the world.
In Origin Story, Sentry is actually winning in his first fight with Alex Harris. That is, until he decides to fire a bolt of energy made of "the power of a million exploding suns" at her. As a Kryptonian, Alex absorbs and metabolizes sunlight to make her stronger, more invulnerable, and heal faster. Oops.
In Opening Dangerous Gates, the party travels to a town and finds a forcefield blocking their path. They destroy it and continue on their way. When they arrive, they find the town burned to the ground. Some demons thank them for getting rid of the forcefield that was stopping them from reaching the town.
In One Foot – Another Multiverse SI Poe does this a lot. Most of his actions tend to be less 'helpful' and more 'why would you do that?!' in nature. Heck, he does it at least once per world. One of the biggest examples is when he accidentally gives Father the formula to nuclear fission while trying to pass the written portion of the State Alchemist exam because he forgot that Amestris is at around World War I levels of tech.
In I Still Haven't Found What I'm Looking For, Harry Potter utilizes magic on a massive scale not seen in millennia to defeat an invading Separatist army and in doing so causes all Force users to become magic users. While the first demonstration of this is a trainee accidentally apparating instead of using a Force Jump, it quickly becomes apparent that Palpatine is far more powerful now as well and is quickly learning what he can do with his new powers.
While Naegi has plenty of these in And Again, Monobear explictedly points out one to him near the end: his decision to send the strongest students off to supervise Mukuro retrieving her arsenal from her room.
Monobear: What a great idea! Send all the strongest of you bastards away! Defend everyone else with the power to sing and have rich parents! What was your talent again, Naegi-kun: Super High-school Level Strategist?
In Graduate Meeting Of Mutual Killing, Tomoe Chiba, ex-SHSL Strategist, was really focused on helping and getting people to help to escape. In her efforts, she gave Matsukaze a signal interceptor for him to use as he wanted. The interceptor would later cause internal bleeding that kickstarted Matsukaze's death. Another action she did was dismantling a Monobear. While it wasn't explicitly said, it's possible that the screwdriver that ended Matsukaze's life was left there by Chiba after being done with the dismantling
In System Restore, in an effort to prevent any murders caused by Monobear's "motive" in the second chapter, the Twilight Syndrome video game that he dares the cast to play, the group comes up with the idea of having people guard the game console. Unfortunately, one combination of guards Hinata comes up with is Pekoyama and Kuzuryuu. Unbeknownst to Hinata, Pekoyama has been raised to view herself as Kuzuryuu's "tool," and as such, the former lets the latter play the game, leading to the murder of Pekoyama, by someone hoping to preemptively stop her from killing anyone else.
In Light and Dark The Adventures of Dark Yagami, the girl from the bus kills Dark and gets hailed as a hero for doing so, but he goes to the Shinigami world after dying, kills Blud and becomes king of the Shinigami. He even thanks her for killing him and enabling his ascent.
In The 10 Doctors, the entire reason the Time Lords lost the Time War is that the Valeyard — who the Time Lords created in the first place — killed Damon before he could give the Daleks' attack strategies to the Eighth Doctor. If this hadn't happened, the war would have been much less destructive, the Time Lords and Gallifrey would still be around, and the events of The End of Time would never have happened. Essentially, it's the Time Lords' own fault they got into the Time War, which isn't too surprising.
Later, the Tenth Doctor finds the relevant data spool. He reluctantly decides against giving it to the Eighth Doctor, in order to keep history on track.
In Outrage, the Sixth Doctor reveals Jem's secret identity to Rio Pacheco. Rio doesn't take it well.
In Seeds of Destruction, the Fifth Doctor lands on a lush garden planet, ignoring Peri's request of Pasadena, California. Said planet turns out to be the deadly planet Kryn.
In A Time To Kill, the Tenth Doctor completely shatters James Bond's cover, and the spy is injured in the ensuing fracas.
In Memento Vivere, a Final Fantasy X fanfiction, Jecht forces the party to flee the battle after he tosses his sword at the Espada, thereby losing the weapon. He does it again when he attacks the Moonflow Shoopuf.
In Chronicles of Harmony's End, Array conspires with Discord to overthrow Harmony (the only force keeping the two of them in line), believing he could destroy Discord himself. He was wrong.
Inner Demons: If Celestia had told Twilight about the prophecy sooner, or broken the news to her differently, it's possible that perhaps Twilight's transformation into the Queen of Darkness could have been slowed or altogether prevented. At the very least, Twilight wouldn't have freaked out and turned Celestia to stone in a fit of anger at the revelation.
Twilight actually pulls this on herself. After the above incident, she convinces her friends to lock her up in the library basement while Rarity searches for a way to stop her descent into darkness and free Celestia. Unfortunately, since Spike is busy helping Rarity and Twilight's all alone, there's no one to wake her up from her nightmares, wherein the Queen persona consumes her completely.
A little later on, Sweetie Belle is on the verge of a Heel Realization, when Rarity's unfortunate wording during a conversation hits an emotional trigger Queen!Twilight had previously planted, causing Sweetie Belle to fully embrace her Face–Heel Turn and reject Rarity as her sister.
Ditzy Don't: Twilight's attempt to help Ditzy by implanting magical copies of the Mane Six's personalities in her head first causes her to outclass Rainbow Dash to the point of her public humiliation, then all the group's negative aspects (Fluttershy's sudden rage, Pinkie's personalities, Twilight's obsessiveness, etc) take over, making her go crazy and chase after Rainbow Dash to force her to let her take care of her.
Rolling in Beaches: Wavedancer taking the Rainbow Pearl out of curiosity (and then keeping it to get close to Spike), leads to it being stolen by the Scaly Back River dragons, who in turn break it and accidentally release the Rainbow of Darkness.
The flashback at the beginning of Atlas Strongest Tournament incorporates the story of Tithonus, with Celestia as the guilty party.
When Rarity saw Ran Biao and Razorwing together, she ran away at the perceived betrayal, which led to Razorwing raping Ran. This led to the breakup of their friendship.
Fluttercruel honestly just wanted to teach Fluttershy something important for her own good, to let her learn the cruel part of nature, but it ends up corrupting her for real as opposed to being just brainwashed by Discord. Fluttercruel is absolutely horrified realizing this.
She also gets Fluttershy, and herself, drunk, which Word of God admits may have had something to do with their behavior following and made Fluttershy more vulnerable to the darkness than she otherwise would be.
While it's Loose Canon, it's implied that the Mane Cast are, unknowingly, being manpulated into commanding Princess Gaia's army via a boardgame they're playing with Gaia herself. If this is believed, Rainbow Dash unwittingly guides Gaia's army into "Gaiafying" Gilda.
If this is true, it is quite possible that Trixie and Twilight Sparkle are unwittingly providing a front for Princess Gaia to brainwash their own mothers into being her knights.
In the aftermath of a battle, your friend-suddenly-turned-enemy lays indefinitely helpless before your group. To help console each other on this tragedy and related circumstances, one mentions that it is not too late to restore or redeem this friend. If you think that taking matters into your own hooves and executing your petrified former friend, Discord, by pulverization is not a logical next step, you are smarter than Galaxia, whose deed costs her life, sets the stage for a 1000 years of tyranny, and worst of all, could have been easily avoided.
During the battle between the Dark World!Elements of Harmony and Discord's remaining minions, Spike has to choose between helping Rarity and Twilight, both of whom got separated from the main group while fighting Fluttercruel and Angry Pie (respectfully). He figures Rarity will need more help and goes after her; in fact, not only has Rarity by this point killed Fluttercruel, but Twilight ends up overpowered by Angry Pie, who rips out her Element and leaves her for dead.
In Secrets and Lies, Twilight tries to prevent Discord from escaping without alerting anypony who doesn't need to know what's going on, in order to avoid triggering mass panic. On top of this, she's keeping other secrets from her friends, and her attempts to fix everything without revealing them lead to terrible complications. Her biggest break, however, is when she determines the best way to help Fluttershy is to have her commited, leading to her getting dragged away kicking and screaming and locked up in a madhouse.
In Screwball Over, Twilight Sparkle and friends discover that their attempts to cure their friend and neighbor Button Stitch of her tendency to turn into her Toon Physics-powered alter-ego Screwball were all part of a plot by Discord to summon an endless swarm of mini-Screwball clones and swamp Equestria. Discord, having been turned back into stone at this point, has left a message mocking Twilight for falling right into his trap with her goody-goody ways. Twilight is only saved from a Heroic B.S.O.D. by the reassurances of her friends and the princesses that she shouldn't blame herself.
Pulled on the readers in Dungeon Master Bonbon. When Lyra temporarily takes over the blog, they spam her with asks saying how Bonbon loves her, causing her to break up with Bonbon out of fear she's leading her on.
In A Stranger Among the Voices, Twilight and her friends have an offscreen incident early in the story that involved repelling a chimera and forcing it back into the Everfree Forest. At this point, the reader can guess this is the same chimera that attacked Dinky in the previous chapter, which wouldn't have happened if Twiilght's timing had been a bit later, as she notes to herself.
The eponymous Villain Protagonist in The Rise of Darth Vulcan main occupation-aside from looting ponies- is calling out the ponies for their dumb actions in canon. In-universe, many of actions of the Mane 6 and others make the situation with him a lot worse:
Twilight tried to get rid of the Alicorn Amulet by dumping it in a void, which allowed it to reach Ted, and gave him the power to become Equestria's next villain.
Rainbow Dash attacked Darth Vulcan immediately when he entered Ponyville because of his hostile nature and evil-looking armor.
The Mane 6 use the Elements of Harmony on him, thinking he was brainwashed by the Amulet. Not only was he not brainwashed, he believes the Mane 6 were trying to brainwash him. In their defense, the Mane 6 didn't realize the true effects of the power of Harmony, something Luna also claims are not pleasant.
Filthy Rich attempted to sell Vulcan out to the authorities and take his bits anyways. This resulted in Diamond Tiara being kidnapped and Ponyville being looted. He was very traumatized by these events.
Luna tried to interrogate Vulcan by beating him up. Not only was she given a brutalHannibal Lecture which drove a wedge between her and Celestia, she also exacerbated the trust issues of what was deduced to be an angry teenager. She is ashamed when she realizes this, but by then it was too late for her to make amends.
Twilight ignored his hatred of mind influencing magic and brought mind-changing spells to his trial. It's no wonder he didn't stick around to help stop Canterlot Castle from falling into a lake.
Celestia allowed the ponies of Equestria to brainwash the thestrals into thinking that their need for blood was a mental disorder when it's about as much a disorder as the desire for water and force them to use a woefully inadequate alchemical substitute. As a result, fifteen thestrals have defected and joined Darth Vulcan. And when the Princess finds out, Celestia flat-out bans her from doing anything about it, destroying the last bit of the Princess's trust in Celestia. All this for no better reason than that the ponies of Equestria find the thestral's need for blood creepy.
Luna pulls this on Celestia after Shining Armor is fired from command of the Royal Guard by inducting him as the leader of her Night Guard, to allow him to secretly continue investigating the conspiracies. Celestia had seen the attack on Shining coming and had been grooming him as a populist voice in the courts, which was central to most of her near future plans, but she didn't tell Luna because they always ended up butting heads over long-term strategy and she didn't tell Shining so he, and their enemies, would believe he was no longer a threat. Not only does Celestia's plan go up in smoke, but she worries that the relatively unsubtle Shining will be caught doing more than is strictly legal for the Night Guard, at which point Luna's crown and authority (and Celestia's own, if she steps in) would be pretty much bucked. Celestia's setup of all this also means that she played this trope on herself, but since Luna acknowledged Celestia might well have a plan but was tired of being left waiting in the dark, she really has no excuse.
Twilight thinks this of herself after her espionage mission triggers a war between Zebrica, Saddle Arabia and Equestria. Considering things had been volatile for a while, and she stopped a cult leader from wiping out the whole Zebrican continent and triggering worldwide damage, no one else agrees.
The Clockwork Consequence... Hoo boy! Let's just say that Twilight's Batman Gambit to escape from Nightmare Moon in the Night Mare Moon timeline from The Cutie Re-mark Part 2 was not one of her better ideas. But, to be fair she had no idea just how far Nightmare would go to protect her rule.
During Octavia's fight with the minotaur in Stroll, she accidentally hits the knob to one of the burners on the stove, setting Sesame Seed's apartment on fire.
Escape From The Hokage's Hat has Shizune not let a recovering Naruto (who just wants to provide support) fight by slapping him with a Interface Screw jutsu. This ends with Hinata (literally) getting stabbed in the back, Shizune too tired to heal Hinata while the enemy attacks, Naruto using a Forbidden Dangerous Technique that could've fried his charka network to save them and her losing Naruto's trust. She, however, at least notes how she screwed up and starts fixing the mess.
Blackkat's Reverse has Kakashi forcefully ripping Naruto from his newfound uncle's arms and refusing to hear Kurama's plea to not separe them. It results in Kurama kicking his ass, abducting Naruto and fleeing Konoha.
Discussed in The Second Try: Asuka and Shinji lived for a while in a barren world because they were unknowingly furthering a "wipe humanity out" scheme when they were fighting alien monsters... and because Shinji was sick of everything. In the fourth chapter, Shinji states that all of humanity is gone because of him. Later on, Asuka and he admit that they were part of the trouble in the original timeline just so much like everything else.
Discussed in The Wrong Reflection. Gul Morag of the Klingon-Cardassian Alliance in the mirror universe blames Starfleet for the resurgent Terran Empire. It's mostly accurate,* although note that Starfleet wasn't responsible for James O'Brien killing Mirror Miles O'Brien and going back to the Bad Old Days but Captain Kanril Eleya refuses to be held responsible for it because all it happened before she was born and Julian Bashir and Kira Nerys were only doing what they had to do to escape Alliance imprisonment. Still, it is the fault of the prime universe Ferengi that the Terrans now have cloaking devices, something which Eleya fixes by giving the Alliance Starfleet's data on how to defeat a cloaking device.
In New Home, Peridot and Steven realize that the Crystal Gems are attacking all the Homeworld grunts that were send to help neutralize the Cluster, potentially dooming the Earth to destruction and rendering Steven's efforts meaningless.
Everything in Imperfect Metamorphosis can ultimately be traced to Team 9, and Cirno specifically. They were responsible for both releasing the Shadow Youkai and getting Yuuka involved, and their constant meddling continued to make things even worse. Other characters caused a lot of headaches too (especially Eirin and Yukari), but only Marisa, due to having stolen the box from Patchouli in the first place, allowing Team 9 anywhere near it, is anywhere near their level of blame.
When Fai finds out about R!Syaoran and Kurogane's Secret Relationship and the fact they were sleeping together in Shatterheart, he forces an ultimatum for Syaoran to either to tell the truth to Sakura or to break up with Kurogane, which he sees as an extremely codependent relationship. This has the side effect of deteriorating Fai's budding reconciliation with Syaoran. His earlier ostracization and hatred of the latter and forcing him to end his only positive relationship proves the straw to break the camel's back and Syaoran wants nothing to do with him. Since Kurogane was the only one Syaoran could turn to for comfort, his PTSD gets worse and he mentally and emotionally falls apart.
Okay, Luminosity's only difference is that Bella's a different character, so naturally the differences for good or ill will be her fault...village murdered because of her? And now the Volturi have shapeshifters on their side? Because Bella thought it would help them. Oops. And Elspeth is following in her mother's making-the-Volturi-stronger footsteps.
With This Ring: It's Martian Manhunter's fault that the Team gets trapped in a mental training simulation due M'gann freaking out because he didn't properly assess her capabilities. It was his fault that he deliberately made the simulation unwinnable that Orange Lantern becomes so desperate to save his friends and the remaining survivors that he summons the Ophidian. In the simulation, Manhunter kills M'gann to end the simulation in front of the Ophidian-infused Orange Lantern. This traumatizes Orange Lantern and cause his mind to fuse with the Ophidian's.