Though to be fair, Saki should have probably stopped them from running away by directly bringing Mamoru back home when they found him in the first place
Gundam 00 gives us a Nice Job Breaking It Villain. Near the end of Season 1, the Big Bad's assassination of the hibernating Aeolia Schenberg triggers a Dead Man Switch which unlocks the Gundams' Super Mode. With this advantage, the good guys have a fighting chance in what had previously been an all but impossible battle. Thus, in the first season finale, They manage to kill the Big Bad and fight the remainder of his forces to a bloody stalemate.
Celestial Being had an unintentional variant of this. Their plan in Season One involved getting the whole world to quit shooting each other, and unite them towards a common enemy (them). Unfortunately, this works far too well as this results in the creation of the A-LAWS.
In Gundam Wingthe five Gundam pilots are planning to killing Earth Sphere Alliance leaders to stop their oppresion of the space colonies. Problem is, they actually started peace talks when the pilots do the plan. Result: peace-seeking leaders killed, OZ and Romefeller Foundation is free from hindrance and take over the Earth, and the pilots get serious cases of Heroic BSOD.
In the original Mobile Suit Gundam, part of the events from before the show starts involves The Federation diverting a Colony Drop from hitting their incredibly well armored fortress in the middle of the mostly uninhabited Amazon rain forest. The problem is, a piece of it hit the very much inhabited Sydney Australia. Oops. To be fair, the chances of it hitting not only a continent, but a major city at that, rather than splashing down in the ocean, are probably pretty slim.
In Gundam Seed Destiny, one might say this trope occurs throughout the series with the Freedom and Archangel interfering with the war. But at the Second Battle of Onogoro, Kira preventing Shinn from continuing his attack on Orb allowed Djibril to escape, thus letting him fire Requiem and destroy a lot of the PLANTs. Shinn in SEED mode most likely would have stopped Djibril before getting on the shuttle or soon after he launched. Kira and the Archangel would have lost Orb, but considering the only Orb forces they had at the final battle were the ones they had before Onogoro...
One could argue that Athrun's attempts to discipline Shinn by slapping him only made Shinn more resentful of him, thus pushing him towards his eventual psychosis.
In Tengen Toppa Gurren Lagann, the first part of the story is about the struggle for humans to return to the surface by killing the evil overlord keeping them underground... Sadly, the evil overlord was protecting the humans from something worse. Ooops.
Turns out that the "Something worse" is actually oppressing and destroying sentient races across the universe because if they become too advanced, they could cause existence itself to collapse.
Some have theorized the opening takes this even farther, and actually shows an alternate ending right before existence does end.
And Rossiu, in an attempt to figure out how close "something worse" is to happening, evacuates humans from their underground cities even when they don't want to leave — not-quite-unwittingly bringing disaster closer with every person living on the surface.
Blood+. By freeing Diva, who had been tortured and experimented on until that point and transformed into a Psychopathic Manchild by the scientists who found them, from her confinement in the tower, Saya set loose the Big Bad who would go on to slaughter humanity for the hell of it for the next hundred-plus years. The entire series revolves around Saya and her pals trying to hunt down and kill Diva to correct Saya's mistake.
Magic Knight Rayearth. Save the princess, kill her kidnapper, Save Both Worlds, right? Uh... not when he was RESCUING the princess from a life as a Barrier Maiden that can't even think for herself, and, because she knew she was stuck in that role and her happiness and self-fulfillment could kill the whole kingdom, she sent the Magic Knights as unknowing Laser Guided Tykebombs to kill HER, not him. After the resulting psychological trauma of The Reveal and being forced to kill the both of them, that leaves the place without a Barrier Maiden, and guess who has to choose between letting the kingdom suffer and dooming one person to a life of misery? No wonder Hikaru decides to create a kind of Merged Reality instead.
Occurs on a spectacular scale in Neon Genesis Evangelion. While Gendo and SEELE propagate the general purpose and technically true cover story that their mission is to prevent the Angels from initiating Third Impact (which would kill the entire human race), the reason for killing the Angels is in actuality so that they don't interfere in the Human Instrumentality Project, which is a fancy name for SEELE-controlled Third Impact.
While Gendo has been preparing his own Instrumentality plan for years under the noses of his superiors, one that is arguably a little better than "genocide and hope for the best," Shinji accidentally triggers their original plan at the eleventh hour. Things get better. I guess.
Played straight in Rebuild of Evangelion 2.0: Oh Shinji, in the short span of 90 minutes you've managed to make yourself likeable again, have reached out to other people, grown a bit of a spine, and even got to kick a gargantuan amount of ass... Too bad saving the girl results in Third Impact, trapping you and her in your "Giant Robot" while your bishounen counterpart descends from the moon to lance you in the chest!
Retconned into Exaggerated territory by 3.0. Shinji actually started Third Impact. Which trapped him in Unit 01 on the moon for 14 years, killing off the majority of the population the Second Impact didn't and being universally hated by everyone who survived. And the kicker, Rei II is now stuck in the core of Unit 01.
It gets worse. In the climax of 3.0 itself, Shinji, aided by Kaworu, eventually sets out to obtain the spears of Longinus and Cassius to somehow Set Right What Once Went Wrong back in 2.0. But as it turns out, they have been fooled big time, and what's waiting for them are to identical spears that will only finish the destruction of humanity by triggering Fourth Impact. Kaworu notices that something's off, but that gets lost in the confusion of the ensuing battle, after which Shinji pulls the spears out... Cue epicMy God, What Have I Done? moment. The icing on the cake? Kaworu, the last person to treat Shinji like a human being, decides to pull a Heroic Sacrifice to end the impending apocalypse, dying violently right before Shinji's eyes. The kicker: It doesn't help.Mari ends Fourth Impact by yanking Shinji out of his mecha. That poor kid just can't get a break.
In Pokémon Special, one of the first things Yellow does is breaking up an attempt by an apparently resurfacing Team Rocket to hijack the ship S.S. Anne. Said hijacking was part of a plan by a reformed Lt. Surge to lure out the Elite Four with a rumor about finding the one they were looking for.
During the Platinum arc, Buck figures that he has to protect the Magma Stone from Team Galactic, so he moves it, intending to bring it home with him. This action in fact awakens the legendary Pokemon Heatran, Team Galactic's true target, and Buck has unwittingly brought them right to it.
In the BW arc, the Dark Stone is in Lenora's impenetrable office and the only key is smashed. However, Black knows how to get in thanks to his Gym battle against her. A Plasma Grunt, disguised as Brycen, tricks Black into opening it for him and knocks the kid out before stealing the Stone.
Digimon Adventure Myotismon leaves the fog that he created behind after he is killed, after which Joe hangs a lampshade on this trope. Venomyotismon shows up just hours later.
Frequently implied to occur whenever the Dirty Pair are on the job (it's the reason for their Embarrassing Nickname), but one OVA episode spells it out plainly. The Angels are investigating the mysterious deaths of several hundred mining employees on a planet run as a religious colony; they find that the religion's leadership has evolved into a murdering cult that, with the help of a ring of weather satellites, is capable of calling down Sodom-and-Gomorrah-style devastation down in a specific location. After they destroy the cult's station in orbit, they assume correctly that the cult's reign of terror is ended. Unfortunately, the space station was also the control for the weather satellites, and the weather satellites weren't just used for destructive purposes; the Angels look down from orbit and see about nineteen hurricanes beginning to form, with no weather-control system left to prevent them...
Princess Tutu: Mostly because the characters are living in a tragedy and Drosselmeyer gets his kicks from making a person's good intentions be catalysts for the demise of those they're trying to help. The main character frequently makes everything worse courtesy of her humble attempts to help Mytho.
In order to prevent Mytho from destroying his own emotion of love, Fakir cuts the mythical sword capable of doing so in half. It also ensured that his heart could not be shattered again. Now how could this possibly be a bad thing? The piece of Mytho's heart that held love was recently bathed in raven's blood, thus corrupting it — and now it's corrupting Mytho. The only way to get the thing out? Shattering his heart by using the very sword that Fakir destroyed. Whoopsie...
Code Geass: In the first season, facing obliteration, Lelouch/Zero geasses Suzaku to "Live!" One year later... Lelouch orders his best pilot Kallen in her new Super Robot to kill Suzaku, which results in the Geass activating, forcing Suzaku to turn to his only remaining weapon: an anti-matter nuke. Nice job nuking Tokyo, anti-hero.
Suzaku is just as complicit in the latter, as he was strongly advised by not only Cecile and Lloyd, but also Kallen herself, to retreat. Suzaku chose to remain there and suffer death, which is what led to his Geass command activating and driving him to fire off FLEIJA. Not to mention that Lloyd strongly recommended him to not equip the nuclear weapon to his KMF. Not following this advice led him to using the nuclear weapon as his last resort.
Suzaku himself is a walking catalog of this trope, trying to be the hero but screwing things up due to his own masochism and that he's playing for the wrong team.
Also after Shirley's, Lelouch has the Geass Order exterminated, while it does damper V.V.'s plans, it actually causes the Black Knights to distrust him further. If he really wanted to eliminate the Geass threat, he should destroyed that one island that was implied to one of several special sites to active the Ragnarok/Assimilation plan, which would have really fucked up V.V. and Charles' plans completely.
Of course, he already did that to the inside of one of them enough so that it would collapse. So, nice job not breaking it hard enough, anti-hero.
This trope could also be attributed to Ohgi and his culpability in convincing the Black Knights to follow his lead in betraying Lelouch due to his gullibility regarding the claims of Schneizel and Villetta, which results in isolating Lelouch once and for all, and him ultimately going down the path of the Zero Requiem, which is largely an excuse for him to die as he has lost all hope, and him causing more destruction than he otherwise would have for a peace that logically won't last very long, as it is built on a flimsy pretext. That the ending is as happy as it is, given the circumstances (Lelouch doing worse things than anyone in history), is pure No Endor Holocaust at its finest.
In Hell Girl: The Cauldron of Three, Mikage is able to sense when someone nearby may soon be a client of Enma Ai. In one episode, she stops one such person and urges her not to use the Hotline to Hell... which she had never heard of until then. Smooth.
In the Season 2 finale of Yu-Gi-Oh! GX, Kenzan tries to stop a brainwashed minion from firing the Earth-Shattering Kaboom by smashing the laptop computer that controls the satellite cannon. As soon as he does this, he learns the missile was just fired, and he's just destroyed the only means of stopping it!
The hero wasn't immune to this either. When Judai was little, he had a card named Yubel that was very protective of him. Whoever Judai dueled or was close to, Yubel would cause harm to them and made them fall into a coma. By Judai's request, Yubel was sent into outer space by Kaibacorp hoping it would absorb "the powers of justice" like his Elemental Hero Neos would. But instead it absorb powers the Light of Destruction and was driven insane. After landing back on Earth, it became Season Three's Big Bad and orchestrated the events that happen to Judai and his friends.
The biggest and baddest of the BigBads in the Zatch Bell! manga by name of Clear Note, who planned to use the King's Privilege to bring genocide to the Mamodo, was powerful enough that it took Zatch and Brago's strongest spells to beat him. And what happens next? It somehow enables him to become powerful enough that Makoto Raiku had to intervene. What's worse, Kiyo somewhat knew what would happen if they did what they did thanks to his Answer Talker ability, which allowed him to determine the answer to any question in an instant, but if they hadn't, Clear would have won anyway.
In Tsubasa Reservoir Chronicle, Sakura leaves one of her memory-feathers behind in Acid Tokyo to keep the reservoir pure and the inhabitants alive. While this act ultimately results in her birth, the aging of the feather also allows the Big Bad to win when he returns it to her body in the "several hundred years later" dimension.
Also, Syaoran. If only he could turn back time, everything would be okay and he could save her, right? WRONG!
This is clearly becoming a theme, since it turns out that the entire plot was set into motion by Clow's split-second wish that Yuuko wouldn't die, though he didn't intend for it to turn out quite that way and the Big Bad just happened to feel the same way, and believe strongly in the tenet of Screw Destiny.
The end of the Setsuna-arc in Ga-Rei: Kensuke is killed, but after a long struggle and overcoming impossible odds, Kagura finally defeats the Big Bad, making sure the evil hate-fox inside her Artifact of Doom never has to be summoned. She then proceeds to summon said hate-fox to revive him, resulting in the destruction of a good deal of Tokyo and nearly killing her in the process. If it weren't for the Power of Love, things could have gotten a lot worse...
A Certain Scientific Railgun: Misaka finishes off the overpowered Kiyama-sensei, bringing an end to the Level-Upper Arc...except now the entire network suddenly becomes unstable without her to control it, and it's just given birth to a weird...fetus...thing, and now it's heading for a nuclear reactor. And the only person who knows what's going on is out for the count.
Angel Beats!: The Afterlife Battlefront spend an entire episode discrediting Angel by making her score zeroes on her exams, and manage to get her fired as Student Council President. Unfortunately, compared to the passive Angel (who's only been reacting to the Battlefront's attacks), the guy who replaces her is a huge asshat, and the entire Battlefront gets wiped out and very nearly obliterated.
The entire plot of InuYasha is based on this. Kagome kills a demon that stole the Shikon Jewel and in the process shatters the aforementioned MacGuffin. This kicks off an incredibly drawn-out plot where the main characters have to fix the jewel and fight evil demons who want its power for themselves. Nice job breaking it, Miko.
Doki Doki Densetsu Mahoujin Guru Guru has the characters discover Kukuri's birthplace where items for her use had been left behind. The bad news is the items had been collected by monsters called Gimu Gimu. Good news is they're tame and don't object to people taking back what's theirs. Unfortunately, they also don't object to people taking items and selling them, which is exactly what Nike, Kukuri, and Toma did prior to reaching the ruins. To make it worse, a necklace meant specifically for Kukuri was lost when Toma earlier used the item it was kept in as an impromptu rocket launcher. Poor Kukuri simply snaps at this point and regresses to a four-year old for a few moments. For the record, they did find the necklace soon after.
Manga example from Basara: when Momonoi is killed and the Red King retakes his city, Tatara and his rebel army plan on using the leftovers of Momonoi's gunpowder to blow up the water supply of the palace, making the Red King suffer. Too bad it results in the whole city having no water left and the citizens, formerly happy to be freed from their king turn against the rebels. A desert city cut off from water, good way to get their support hero.
In the Berserk manga, Skull Knight manages to slash Femto from behind at the very moment of his apparent ascension, using his dimension-crossing sword to teleport and get the drop on him. Unfortunately, Femto then redirects his strike towards the vortex of demonic power they are both standing on, and uses its dimension-bending to bring the unholy nature of that place closer to earth, thus bypassing the walls between Seen and Unseen by folding time and space, resulting in the energies of the Unseen manifesting physically on Earth. You've brought about Hell on Earth, Skull Knight. Whoops. Guess that's what you get for bringing in sci-fi conventions into a Medieval Fantasy world!
An earlier Berserk example: when the King of Midland has Griffith jailed and tortured for deflowering his daughter Princess Charlotte, Griffith accuses him of being attracted to her, since she looks just like his dead wife. Now, if those desires did exist, they were only subconscious — the king would probably never have realized they were there. But Griffith's accusation makes him dwell on his daughter to the point of madness, and in one horrible moment he forces himself on her. She stops him and he regains his senses, but the damage is done. Charlotte is traumatized; the king sinks into despair and insanity; and the Band of the Hawk, now the king's only targets for revenge, are hunted for years by Midland's army and worse. Good one, Griff.
In a way similar to the Magic Knight Rayearth example above, the entire first half ofKamikaze Kaitou Jeanne was this. Maron/Jeanne is helping out God by sealing demons to keep Satan at bay and her boyfriend/arch enemy Chiaki/Sinbad is an agent of the devil, right? Bzzt, wrong! It is revealed that Maron's angel sidekick Finn Fish is actually working for the devil and had been lying to Maron the whole time, and Maron was actually helping the devil and Chiaki God, not the other way around. So Maron had actually been helping the devil that whole time, making him stronger than ever.
Urahara guessed that Orihime's unique powers would be of great interest to Aizen so took her off the battle lines to prevent her becoming a target. Instead, it made her more vulnerable as Rukia's attempt to help her develop a use for front line battle made it easy for Aizen to have her kidnapped and Urahara himself failed to keep a close enough eye on her to prevent the kidnapping from happening. The closest thing to a saving grace is that Orihime was actually kidnapped in the Dangai, a space between worlds that is designed to keep the worlds separate and which cannot be broken into. As the man capable of predicting and countering everything Aizen does, it should have occurred to Urahara that the Dangai might just be the best place in the world for Aizen to stage a kidnapping, precisely because people travelling through it are so vulnerable to something as unexpected as a break in... and Urahara knew it was possible to break into the Dangai, because he was the one who'd found a way to do it.
Orihime's kidnapping led to Ichigo's group defying Soul Society's orders and invading Hueco Mundo to save her. Yamamoto, who had hoped to be able to send a captain-class invasion force in three months time was forced to speed up his plans to get the captains into Hueco Mundo to follow after the kids. Then it's revealed this was Aizen's plan all along, knowing Orihime's unique powers would force the shinigami to act to prevent her power being used by Aizen. Aizen successfully split the Gotei 13 forces in half, enabling him to have to only fight half the Gotei 13 at Karakura Town.
In the filler Zanpakutou Tales arc, Ichigo chases Muramasa into his hideout, locates the imprisoned Yamamoto and fights Muramasa to free Yamamoto from the barrier he's trapped in. The end result is that Ichigo's great power does indeed break the barrier surrounding Yamamoto, freeing him. Unfortunately for Ichigo, this was actually Muramasa's plan. The barrier had been created by Yamamoto to prevent Muramasa from being able to brainwash his zanpakutou. Muramasa needed to combine his power with Ichigo's power to obtain the strength required to break Yamamoto's barrier, which Ichigo unwittingly provided him with. The first thing Yamamoto says when the barrier breaks is "Looks like we lose this round". Whoops.
Yamamoto pulled out his Super Power Lottery bankai to fight what he thought wasThe Emperor of the Vandenreich...only for the real one to come out of nowhere and steal the most powerful bankai in existence. Nice job, old man.
In Kinnikuman, all the choujin were celebrating Kinnikuman's victory in the Choujin Olympics. At one point, they toss him so high into the air he goes flying into outer space and bumps into a satellite. As it turned out, that satellite was a prison for the Devil Choujin and that bump hit the release button.
The Patlabor TV series features a pretty epic one, with a simple Honor Before Reason decision in an early throwaway episode snowballing into an increasingly deadly story arc that would dominate the later half of the show. Captain Nagumo is opposed to using the prototype Patlabor SRX-70 Saturn created by Schaft Enterprises because she knows they're going to use the motion data from the police's skirmishes to develop military mechs. All well & good, but because Nagumo threw the Saturn away, Corrupt Corporate Executive Utsumi decided to get the Patlabors' data another way, namely smuggling pre-production military Labors into Tokyo & causing havoc in the streets to draw the Patlabors out.
In Mahou Sensei Negima!, all of Ala Alba went to the Magical World, including the unbeknownst-to-them-Princess Asuna. Asuna is then kidnapped by Fate and the Cosmo Entelecheia remnant, who do... something very ominous relating to her Anti-Magic powers. The going theory is that by bringing the Princess to where the bad guys can get her, they directly caused the revival of the Big Bad. Good going.
He makes a very similar (and also understandable) mistake later: While fighting a Level 4 Akuma, in order to trap it, he deliberately makes himself a target, then summons his sword from across the room, impaling both the Level 4 and himself. This would have worked fine if it wasn't for the little snag that he's the host for the Fourteenth Noah. Even the Akuma looked terrified out of its mind before he managed to suppress it.
Kanda, in the most recent chapters, when he goes berserk and impales Allen with Mugen (his Innocence-forged sword), which causes Allen to collapse and his skin to change into that telltale Noah gray color. The Millennium Earl is positively dancing with joy when this happens, and gleefully thanks Kanda for fully awakening the Fourteenth. Oops....
In Deadman Wonderland, Ganta stops the Deadmen from wiping out the Forgeries because one of his friends was among them. In doing so, he inadvertently incapacitates all of the prison's strongest Deadmen leaving the Forgeries free to start picking them off one by one. As a result, Ganta is completely alienated by his friends.
In Higurashi: When They Cry, the audience doesn't find out until Tsumihoroboshi-hen that Keiichi's actions during Onikakushi-hen were basically part and parcel of this. Granted, stuff was broken. Just different stuff.
Way to go, Nausicaa. In the manga version, you've saved your people, but you've also doomed humanity by destroying the pure, corruption-free genes that have allowed humanity to survive. She believes in the natural cycles, but what she fails to realize is that there will BE no cycles for mankind if they're all dead.
In the Eureka Seven movie, nice job flooding the entire Earth, destroying everything and drowning humanity in the process of saving your beloved Renton, Eureka. Good thing that she lost all her memories aftermath, or else she can't live a guilty conscience free Happily Ever After life with Renton.
In Fullmetal Alchemist: Conqueror of Shamballa, Al opens the gate between his world and ours as the Thule Society does on our side, in an attempt to rescue his brother Ed. Though Ed escapes, it also allows the Thule Society to launch a full scale war on Amestris, and nearly destroys the capitol city
In YuYu Hakusho, the Chapter Black saga, Yusuke allowed Shinobu Sensui to fully power up so they could have a good battle. This ended up getting him killed. Dangit, Yusuke, you made Kuwabara cry again!
Of course this led to a Nice Job Fixing It, Villain moment as Yusuke was resurrected as a mega-kickass demon, but he didn't know he was a hidden demon at that time.
Don't forget that the whole arc was just Sensui's plan to get himself killed by a powerful demon as justice for killing countless demons. Unleashing Hell on Earth against what he sees as a corrupt human race is just frosting added by an enraged Kuwabara. Nice job, dimwit.
In Naruto, Itachi, ordered to kill his entire clan to prevent a bloody coup that would kill all of Konoha, couldn't bring himself to kill his younger brother, Sasuke. As a result, he spared said person, and tried to enact a plan that would end with that person a hero beloved by Konoha, and capable of defending himself and it against the Big Bad. Instead, it ended with him as The Dragon to the Big Bad, swearing to destroy Konoha. Oopsie!
Itachi's entire plan for Sasuke is a Nice Job Breaking It, Hero. It never occurred to him that mind Raping him twice would be detrimental to Sasuke's mental health. Nor occurring to him that beating him half to death twice would make him psychotic. Nor occurring to him that Sasuke might kill Naruto to get the MS. Nor tried to stop Orochimaru from getting Sasuke. If he didn't intervene or interfere with Sasuke's life at all, he would have gotten what he wanted-a Konoha Aligned Sasuke killing him instead of a fucked up mentally Neutral Sasuke.
Jiraiya's desire to give Sasuke a shot at killing Itachi in the hotel is understandable, but it takes all of three seconds for it to become apparent that he's absolutely no match for his older brother. It still takes him at least another five minutes, many broken bones, and a vicious mind raping to actually intervene. In fact, it's very possible that the only reason Itachi even resorted to Tsukuyomi is because he was waiting for Jiraiya to give him the excuse to flee, and nothing else was making him act. This loss was one of the biggest reasons for Sasuke's defection.
The Uchiha's planned coup in the first place could have only ended in tears. And assuming that Madara is not lying through his teeth about wanting to restore his clan's position of power and prominence, it's hard to see how reducing the clan to a few hated, emotionally broken criminals could result in this, though I suppose it's stretch to apply "Hero" here.
Tobi's not even Madara, and actually hated the Uchiha Clan with great vigor. The actual Madara has been dead for some time, and Tobi is really Obito. Which leads to another offender-Kakashi. Why did Obito become a mask-wearing psycho terrorist who's obsessed with his Assimilation Plot? Because Kakashi killed Rin die. Though, to be fair, how was he supposed to know that his supposed-to-be-dead best friend would become said mask-wearing psycho terrorist who's obsessed with his Assimilation Plot, just because the girl he had a crush on died, anyway?
It's later revealed that it wasn't Kakashi's fault. Rin practically forced him to kill her.
Kakashi could have averted a whole lot of this if he'd stayed as a stoic Jerkass and stuck to the damn mission instead of saving his team.
And then there's the Sage of the Six Paths, in his day he saved the entire world from a monstrous threat, founded the Shinobi way, and was pretty much revered as a Messiah and Physical God. That isn't an exaggeration, the threat he defeated was a godlike Ten Tailed beast, which he then proceeded to seal into himself, inadvertently giving himself Reality Warping Powers. He then proceeded to use those powers to create the moon in order to seal away the beast's body and then split it's chakra into the nine Tailed Beasts. Nice guy. Unfortunately he's also indirectly responsible for The Uchiha massacre and the Fourth Shinobi World War. Not to mention all the deaths the tailed beasts caused over the centuries.
Gaara probably wouldn't be half as screwed up as he is if his father had not only had him turned into a Jinchuriki, killing his mother in the process, but also sent Gaara's uncle on a suicide mission and ordered him to tell Gaara that no-one loved him in order to test Shukaku's power. OK, so the Fourth Kazekage was doing it to protect his village in desperate times, but he ended up with an unstable, Ax-Crazy child who kills people left, right and centre, is treated like a monster, has his own siblings terrified of him, and probably would have gotten a lot worse if Naruto hadn't showed up.
Naruto letting Sasuke go at Valley At the End. Instead of going for debilitating blow despite his threats of breaking his arms and legs. He goes for Sasuke protected forehead.
The Second Hokage was of the opinion that the Uchiha would eventually betray Konoha. Well, he certainly laid the groundwork to making sure they would.
Edo Tensei. Even Orochimarucalls him out on documenting a jutsu that allows the dead to be resurrected and used as an indestructible army.
The First Hokage for stopping Tobirama from killing Madara.
Naruto and Minato stopping Kakashi and Sasuke from killing Obito. Ends up with Madara being revived.
Much later, during the Sisters arc, Touma fights Accelerator, which turns out to be the boost he needed to reach Level 6. Thankfully subverted when Touma managed to fix the problem by knocking out Accelerator before he manages the shift.
In New Testament, Ollerus tries to halt Othinus's plans for absolute power by using a transformation spell to remove her Magic God powers, making them fail 100% of the time instead of the usual 50%. Othinus just laughs and reveals that Ollerus basically just handed her omnipotence on a silver platter: if anything she tries is going to fail, then she just has to fail at failing, and she can pretty much do whatever she wants.
In The World God Only Knows manga, Haqua released Fiore out of mercy when Fiore showed "evidence" that she is innocent. Later on, Nora tries capturing her but both Haqua and Nora were restrained. So yeah, Haqua, maybe you should have followed Keima's instructions. Nice job breaking it, Haqua.
Jack is indirectly responsible for Carly dying and coming back wrong; while he left her to protect her from the Dark Signers, he failed to explain that, and was so brusque about it she decided to investigate them to prove she was useful. This led to Divine killing her and her subsequent resurrection as a Dark Signer...whom Jack hadtokill.
Shiranui suffers through a particularly horrific one in the early chapters of Kagerou-Nostalgia. He and Kazuma are staying with a woman whose husband left, planning to travel to the capital and assassinate GeneralKiyotakaKuroda, leaving her and their daughter behind. The husband subsequently returns with a band of mercenaries and massacres the town; when his wife confronts him and accuses him of People Hunting (a form of massacre endorsed by Kuroda), he does not seem to understand what she is talking about. Shiranui realises that the man has been tainted by the darkness in the capital and purifies him. ( Horrified by what he has done the husband promptly murders his wife and daughter, and then commits suicide while Shiranui looks on, unable to bring himself to stop the killing by shooting another human being.) Nice job breaking it, priest.
While they can't be fully considered "Heroes" the Marines in One Piece cause this following The Whitebeard War. Their long, elaborate plan to kill Whitebeard worked, and they also killed off the son of the Pirate King (Ace). The only problem is that in doing so, they horribly screwed up the power balance of the New World, causing several towns to get overrun by pirates. Also, since Whitebeard confirmed that One Piece existed with his dying breath, the piracy rates have practically DOUBLED. To make matters worse, Blackbeard managed to free some of the worse criminals in the history of the world, added them to his crew, and has now stolen Whitebeard's Devil Fruit, which is capable of causing untold carnage, before finally declaring war on the World Government. And to top it all off, the Elders decided to censor the escape of even more of the Level 6 prisoners, meaning they can do whatever the hell they want. So basically, they wound up screwing up the Entire World thanks to a horribly short sighted plan. BRAVO. *Clap* *Clap* *Clap*
The actual protagonists fall into this on occasion as well. Luffy's jailbreak at Impel Down let a bunch of inmates out of prison. Granted, it's reasonable to assume that a fraction of them were actually decent guys like Mr. 2, Jimbei, and the Newkama pirates, but the majority was made up of dangerous criminals. Also, Franky blew up the labrotory of the greatest scientist in the world... twice. He saw the Big Red Button with the skull 'n crossbones on it and thought that it must be a pirate button.
If Puella Magi Oriko Magica is to be believed, then if Madoka didn't try to cheer Mami up in Episode 3, she wouldn't end up being horribly headchomped by Charlotte and instead handily beat her. However, Mami shares blame because she chose that moment to reveal that she needed cheering up.
Madoka nearly killed Sayaka by throwing her soul gem down the bridge in episode 6, and Homura had to clean up after her mess. Again, however, this is due to Sayaka and Kyoukou insisting on fighting each other.
The reason Madoka has so much magical power potential is because Homura attaching additional karma from alternate timelines to Madoka everytime she resets time to save Madoka.
"Excellent work, Homura Akemi. You just made the made the most powerful witch ever."
And again, for much the same reason. If Kyubey had remained silent and didn't explain to Homura that she was the sole reason Madoka was in danger again, the Incubator's plan would have gone off without a hitch and they'd eventually control the Law of Cycles. Instead, he provides the impetus for her to take another option, trapping herself in her own labyrinth for eternity rather than letting the Law of Cycles make contact.
In Puella Magi Kazumi Magica, poor, poor Pleiades Saints. Their plan is to hide Kyubey from existence, making it unable to contact and contract other girls, including preexisting Magical Girls. To help this, they make Jyubey, an artificial Incubator, who supposedly can clean darkening Soul Gem. Only, there is a slight problem: It can't. It can only polished them. Whoooops.
Early in Summer Wars Kenji gets a text message with a math algorithm challenging him to solve it. He spends all night doing so, and when he wakes up finds that the online world OZ used by most of the world has gone haywire and that the algorithm he solved was in fact the company's strongest security code. Ultimately subverted, though, in that over fifty other people made successful attempts on the code and Kenji actually made a mistake in his resulting in it being incorrect.
Doraemon should have a lot of examples throughout the show's run. It at least happened in the movie, "Nobita in the Wan-Nyan Spacetime Odyssey". Nobita finds stray dogs and cats and wants them to have a home. The gang time traveled them back 300 million years and providing them with evolution and a food making machine. When they visit them again, they encounter a time disturbance causing them to be stranded in a different year in that era with advanced sentient dogs and cats, thus started the movie.
In Tsukihime, if it weren't for Shiki "killing" Arcueid, Arcueid could effortlessly defeat all her enemies and Shiki himself wouldn't be dragged into her business.
Actually, this isn't quite accurate. In the sequel story, Melty Blood which takes place 1 year after the end of Tsukihime, its shown that given enough time Arcueid would eventually regain her powers. Its just that the entire Tsukihime story takes place over the course of a few months and Arcueid spends most of that time either hanging out with Shiki or tracking down and fighting enemies instead of resting.
Several of Makoto's leaps in The Girl Who Leapt Through Time would unintentionally break something else while trying to undo something she had broke due to leaping for her personal gain. One example is Makoto swapping places with another classmate in Home EC, causing him to cause the accident in the room that Makoto would have caused. However, this accident would make said classmate a target to bullies. When her friend Chiaki confesses to her, she uses her time leaps to avoid talking to him and he became frustrated with her and eventually decided to date Makoto's friend Yuri. Which would incidentally lead to Makoto becoming jealous and realizing her feelings for Chiaki. Later, when the bullied classmate snaps and throws a fire extinguisher at Makoto, Chiaki attempts to shield her. Makoto then time leaps to tackle him out of the way ... only for the fire extinguisher to hit her friend Yuri instead.
Saint Seiya: Oh all the places to lock your Evil Twin brother Kanon in as punishment for his evil, you shouldn't have chosen the Drowning Pit under the Sanctuary! Because if you had not done that, then Kanon wouldn't have almost drowned and fallen in a hole thrrough the rock wall... which took him towards Poseidon's realm, and allowed him to become the Man Behind the Man in the Poseidon saga. ... Ooops.
In Haou Airen... Great, Hakuron. When your girlfriend Kurumi tried to run away and befriended this kid named Kaafai, you reacted via raping her in front of him and then letting him live. That's not gonna backfire in the future, huh? Well, guess who is the kid who has just shot you to death in your wedding day? Yup, it's Kaafai himself, the same boy whom you stupidly allowed to live.
The backstory to HeartCatch Pretty Cure! is this: Yuri and her fairy partner, Cologne, realize that the Desertians are getting stronger. With two new fairies, Chypre and Coffret, around, Cologne urges Yuri to use them to find help, but she declines, not wanting to burden others with the job and instead decides to try to obtain the Heartcatch Mirage. When she does so, the Dark Precure is waiting for her at the Precure Palace and Yuri tussles with her, leading to Cologne's death, her depowering and shouldering the responsibilities of saving the world to an Innocent Flower Girl and her Genki Girl best friend with no brain-to-mouth filter.
The same goes with Yes! Pretty Cure 5 as it was Coco's kindness that allowed Nightmare to get into the Palmer Kingdom and destroy it.
Reiner and Bertolt from Attack on Titan might have been captured without incident if Eren hadn't mentioned their village at that particular moment. Instead, it leads to Bertolt suggesting they should go home and Reiner confessing the truth to Eren, causing things to rapidly go downhill from there.