However, as a result of releasing Demon King Piccolo, Pilaf also ensured that Goku would grow up to be a warrior trained by Kami himself, and thus be powerful enough to put up a fight against all the increasingly-stronger Big Bads that came to Earth afterwards, up to and including Beerus, the God of Destruction. So, uh... thanks for indirectly saving the Earth, Pilaf?
Remember how, in Dragon Ball Z, Majin Buu's evil half got unleashed, exterminated the entire human race, killed off half of the Z-Fighters and then finally blew up Earth? It all happened because of two crazed gunmen in an episode aptly titled "The Evil of Men", where they shoot Bee (Buu's pet puppy) and Mr. Satan (Buu's new best friend who was in the process of rehabilitating him). As a consequence, Buu snapped, released Evil Buu, who then proceeded to absorb him and transformed into Super Buu; the rest of the Buu Saga is the entire universepaying the ultimate price for what those two guys did.
While Goku, by his own admission, could have killed off Majin Buu whilst in his Super Saiyan 3 form, he deliberately chose not to because he wanted to give Trunks and Goten a chance to do so and because he didn't want the next generation of Z-Fighters to think they could just depend on him all the time. While Goku does have somewhat of a point, his inaction still led to the creation of Super Buu, who ends up killing off most of the Z-Fighters.
On another level, him holding back during the fight against Majin Vegeta caused the latter to believe he could defeat Buu and led to him blowing himself up in a failed attempt to kill Buu. Understandably, Vegeta makes sure to chew Goku out over this when he's revived near the end of the Buu Saga.
When Super Buu enters the Room of Spirit and Time to fight with Gotenks, the fused Saiyan deliberately downplays his power, even verbally stating that Buu is overwhelming him. This causes Piccolo to blow up the entrance in an effort to trap Buu forever, at which the horrified Gotenks reveals he was faking all along. For the icing on the cake, Buu promptly uses his ability to scream open dimensional doorways to escape, and by the time Gotenks is able to replicate the feat, Buu has already murdered everyone on Kami's Lookout. Because of this, none of the Z-Fighters trust Gotenks as a useful fighter ever again.
Dragon Ball GT: As revealed during the Shadow Dragons Saga, the Dragon Balls produce negative energy for each wish granted, which is stored in the balls for it to dissipate harmlessly over 100 years; in fact, the Dragon Balls scatter after every use because it's expected to take years to find one through conventional methods, thus allowing them to disperse the negative energy harmlessly. Unfortunately, that countermeasure was rendered completely useless when Bulma invented the Dragon Radar, allowing the Z-Fighters to find the Dragon Balls very quickly and summon Shenron numerous times, eventually leading to the birth of the Shadow Dragons.
Misa Amane from Death Note does this twice, to the severe detriment of both sides. Once, it's as a Poisonous Friend to Kira, giving L physical evidence in the case, and the other time, it's out of Mad Love to same, endangering her own life to the point that her shinigami has to intervene. For a fervent Kira supporter, she's really a bit of a loose cannon: she never seems to fully appreciate her role in either disaster.
Sachiko asks her children to bring their father a change of clothes, and Sayu is reluctant to take the task. When Light volunteers, he ends up running into Naomi, learning about how close she is to figuring out an important part of how he kills people, and silencing her before she can reach L.
Aoba Yamashiro. Don't know him? He's a recurring background ninja in Naruto who was probably never formally introduced, databooks aside. Why is his name so well-known in some parts of the Naruto community? A single careless sentence out of Aoba's mouth revealed Itachi's whereabouts and goals to Sasuke. Following that, Sasuke tried to beat up Itachi, was soundly defeated, and pondered why Itachi would be interested in Naruto. Sure, Sasuke wasn't particularly stable before, but this huge blow to Sasuke's self-esteem was the catalyst for the event that would finally send Sasuke over to The Dark Side.
In an early Pokémon episode, Ash is right on the verge of capturing a Weedle, when he is rudely interrupted by a samurai, who challenges him to a Pokémon match. During said match, which is a Mirror Match between their two Metapod taking Harden commands, the Weedle manages to escape back into its tree and alert a swarm of Beedrills, who proceed to capture Ash's Metapod. In one of the series' earliest Broken Aesops, Ash is forced to learn a lesson about not making excuses about not finishing what he started, even though it was the fault of the samurai, who berates him for all of this, that Ash wasn't able to finish in first place, all because he didn't have the courtesy to wait until Ash was done. That being said, the following battle with the Beedrills causes Metapod to evolve into Butterfree.
Hitomi Shizuki of Puella Magi Madoka Magica. There's this friend of hers, Sayaka, who likes Kyousuke, who lost the use of his arm in an accident. Through a Deal with the Devil, Sayaka becomes a Magical Girl and heals Kyousuke's arm. Hitomi, who has no idea about this, also loves Kyousuke and decides to confess to him, but not before offering Sayaka the first shot. Sayaka doesn't act on the offer, because she just found out that magical girls are liches, so she sees herself as unworthy of Kyousuke's love. Her depression only worsens when she sees Hitomi confess to Kyousuke, and because of the nature of the aforementioned Deal with the Devil she becomes what she fights. A third party, Kyouko, self-destructs to Mercy Kill Sayaka. In short: Because of one confession, two girls died. Now, poor Hitomi did the best she could with what next to none information she hadand there's also how other girls would've gone after Kyousuke behind Sayaka's back and without anything resembling respect for her feelings (while Hitomi acknowledged them and actually told Sayaka to go for it first), yet the fansstill hate her. At least she isn't an Idiot Houdini, according to her and Sayaka's teacher Kazuko: After they found Sayaka's lifeless body, and after her funeral, Kazuko tells Madoka's mom Junko that Hitomi feels responsible in some way, and Word of God all but shouts that a wavy-haired girl seen sobbing her heart out in Sayaka's funeral is poor Hitomi. Cue the Fridge Horror when you realize that, due to both having been Locked Out of the Loopand the lack of info regarding her death in general, it's a sure thing that poor Hitomi believes she unknowingly drove poor Sayaka to suicide).
Some may consider Madoka to be one. Had she not tossed away Sayaka's Soul Gem, she wouldn't pass out/go all corpse-like and Kyubey wouldn't have spilled the beans, thus giving Sayaka that state of mind that she's no longer human. Although to be fair, her mother also falls into this line for giving her that idea. But really, Kyouko is to blame: had she not challenged Sayaka to fight, the already emotionally-unstable Madoka wouldn't had flipped her shit and tossed the Soul Gem to stop the eventual fight.
In Bleach, Uryu's use of hollow bait to start a hollow hunting contest with Ichigo has the unintended side effect of also drawing the attention of the Soul Society, enabling them to track Rukia down.
In the original game, this also happens - but one can control Makoto's action and make sure things won't be as bad. Or make them even worse
In Bakuman｡, one factor that contributed to Detective Trap losing popularity and being cancelled is two other similar manga - Phantom Thief Cheater and Detective Gosuke Akechi - both of which were submitted while Mashiro was in the hospital and on hiatus, and debuted after Mashiro's release.
Neon Genesis Evangelion. The pilots of Evangelion Transport Neopan 400, the American Weather Bureau and the technician at Airbase Ekta 64 are never named, are never spoken of again, but ALL are responsible for the entire Bardiel Incident and Cerebus Syndrome the show developed towards the latter half. This might not seem obvious at first, so here's the chain of events: Due to the unnamed characters deciding to fly through an odd-looking cloud, Bardiel manages to corrupt Evangelion Unit-03 on its way over to Tokyo-3. This leads to Shinji crippling his best friend for life, resulting in his resignation from NERV. Because he resigned and Unit-00 was damaged in the battle, Asuka has to fight Zeruel alone (which ends badly and results in her sync ratio collapsing). Shinji is also absorbed and Unit-01 awakens. This leads to Asuka getting mind-raped into a coma, with Shinji unable to assist due to the freeze on the awakened unit. Due to the above events, Rei is forced to go out alone against Armisael, resulting in her being blown to pieces and Gendo replacing her with a soulless clone. This causes Shinji to go into mental freefall, only to be assisted by Kaworu... whom he has to kill as he's actually the 17th Angel. Cue Shinji having a Heroic BSOD, as his new friend/prospect-Gay Option is dead, his maybe-Love Interest is in a coma and Rei is back to being an emotionless pawn in Gendo's plans. All of this leads to Asuka being forced to fight the MP-EVA series without support, resulting in her Cruel and Unusual Death. Shinji's mind breaks at this, and Third Impact is triggered, resulting in almost all life on Earth being destroyed. The result of not going around the suspicious-looking cloud: The End of the World as We Know It.
Takehiko Henmi from Oniisama e.... How so? Six years before the story started, he visited his best friend Takashi Ichinomiya's house and befriended his little sister Fukiko, who unbeknownst to Takehiko got a huge Precocious Crush on him. During these days they spent lots of time together, and she made him promise that he'd attend her birthday party and hear her play the violin. However, Real Life interfered: Takehiko couldn't get to the Ichinomiya mansion in time and Fukiko was completely crushed, since she had practised very hard and also did whatever she could to be the best for him. In any other series, Fukiko would've eventually gotten over it... but in this one, she became a Yandere over him, and that brought LOOOOTS of Wangst and drama for everyone else.
In Riyoko Ikeda's other manga, Claudine, we have Rosemarie. What did she do? Depressed because Claudine only considers her his Unlucky Childhood Friendand she has just found out that Claudine's crush Cecilia is actually the lover of Claudine's *father* Auguste, she mentions this very complicated deal to her Hot Teacher, Louis, who is also Cecilia's younger brother. Little did she know that Louis also had been Auguste's lover in the past... and that he would go Yandere on Auguste and Cecilia. The result... The maddened Louis burns down Auguste's hunting shack, killing both Cecilia and Auguste and then running away while Laughing Mad, leaving Claudine's family without its patriarch. Oh, and unlike other instigators, Rosemariedoes suffer massive consequences: not only she feels horribly guilty for the results, but has half of her face horrifyingly scarred due to barely surviving the fire itself.
Satoshi Oginome from Mawaru Penguindrum. Had his estranged daughter Ringo not seen him with his girlfriend and her daughter, she would've likely not sped up her "Project M" to bring her family back together via the Attempted Rape of her crush, with all that happened later.
Inverted with Isumi in Hikaru no Go. If he hadn't insisted on playing Hikaru, Hikaru would never have discovered that Sai is still present in his Go, would not have started playing again, would not have become the "second genius" and, therefore, would have caused the thousand-year plan of the god of Go to fail.
In Digimon Savers pretty much every bad thing can be blamed on one villain, the Final Boss is just reacting to events. Drasil is such a self righteous blowhard that many people hated it just as much as the guy responsible for everything though, especially since much drama could have been avoided if Drasil had bothered to get off its throne years ago.
Medaka Box: Onigase, and by extension, the Public Morals Committee. Their antagonism towards the Student Council ends up getting Medaka and the other members dragged into the Flask Plan. In addition, Medaka's defeat of Unzen makes her an ideal candidate for the project and thus a target.
In Magical Girl Lyrical Nanoha As, Suzuka serves as one. She, not knowing about the Wolkenritter’s true nature or activities despite having met them as well as Hayate, plans to pay Hayate a surprise visit along with Arisa, Nanoha and Fate on Christmas Eve in Hayate’s. Nanoha and Fate, seeing the Wolkenritter with Hayate, then realize that Hayate is the true master of the Book of Darkness that they are searching for, and are forced into a confrontation with the Wolkenritter, who wish to prevent Hayate from being exposed. Things quickly get worse from there.
In Mai-Otome Sifr, a cat wandering out on the hotel balcony leads to Sifr trying to retrieve it, and falling and having to be rescued by Lena, who berates her for going outside and risking being discovered. It's revealed that one of the poolgoers was an undercover Schwarz agent, and that night, a large Schwarz force attacks, succeeding in kidnapping Sifr with help from M9.
Michiru Saotome from Getter Robo Armaggedon. She's infected with The Virus, which is pretty much a death sentence, and chooses to commit suicide via screwing up with a test flight before it takes over and subjects her to a Fate Worse Than Death. While such a situation is really bad and one can undestand the person's reasons, there are even worse things about it: her father, Dr. Saotome, mistakenly believed that the Getter pilots were to blame, leading to him falling into insanity and becoming a Mad Scientist... which is followed by his murder, Ryoma's imprisonment, Saotome being Back From the Dead and completely Ax Crazy, and the invasion. Needless to say, It Got Worse. (Not to mention her sibling Genki, who witnessed Michiru's death in first row, is horribly traumatised to the point of becoming a Creepy Child).
In Fullmetal Alchemist, Heinkel manages to corner Pride, only for some residents of the nearby slums to come by with sources of light. This enables Pride to set fire to the brush nearby, regain use of his shadow powers and severely wound Heinkel.
Kalos Eidos from Kaleido Star believes himself to be one of these. Ten years ago, he asked a famous acrobat and close friend of his's named Aaron if he truly was up to performing the very dangerous Fantastic Maneuver, a very risky acrobatic trick that had caused more than one death... and few afterwards, Aaron failed to do it and died. This is the reason why, a decade later, Aaron's son and former Kaleido Stage Ace Yuri is pissed off at Kalos enough to try buying out the Stage as revenge... and Kalos does nothing to stop him since he thinks it's his punishmentfor, in his view, letting Aaron die.
While Akito Sohma's father Akira had the best intentions at heart, in an attempt to cheer up a saddened Akito, he told her that she was a special child who was destined and born to be loved. After his death and due to her mother Ren's abuse, Akito ended up clinging to Akira's words and twisted them into one of the reasons why she treats the Cursed Sohmas the way she does - they're all destined to be by her side forever, so she's entitled to do anything to keep them from leaving. And when Kureno's curse disappeared, things started going From Bad to Worse. . .
The aforementioned Ren Sohma also has a place in the trope. She once asked a young Rin if she and her family actually were as happy as they seemed, leading Rin to ask similar things to her parents and to them cracking and abusing her onwards.
Kureno Sohma became one later. The also aforementioned Rin saw him talking to Tohru from a distance (during the time he told her that Akito was a girl) and noticed that Tohru was crying. Since Rin has a Big Sister Instinct for Tohru, she decided to look for Kureno and confront him on making Tohru cry. . . and THAT was when Ren caught her and convinced her to be her accomplice in her plans to steal Akito's treasure. . . leading to her being caught, having her hair cut off, and being locked up in the Cat's Room by the absolutely enraged Akito. . . To be fair the guy sincerely had no idea about Rin seeing him and Tohru, and later he finds out about Rin being "captured" and releases her.
The biggest one, however, is the original God. He was a kind yet lonely man who simply wanted to live forever and for his animal friends to stay with him through eternity. . . but when he made an immortality elixir for that, one of said animal friends (the Cat) disagreed, this sparked quite the fight between them, and thus the Hereditary Curse of the Sohma clan was born.
