History UnwittingInstigatorOfDoom / AnimeAndManga

** In ''Anime/MacrossPlus'', the local PlayfulHacker Yang tries to hack Sharon's A.I. during her concert in OVA 2. [[spoiler: Not only he breaks through Myung's concentration when she's managing Sharon's A.I., but it causes Sharon to see Isamu (who ''happens'' to be Myung's NewOldFlame that she still loves) for the first time... right when her other creator, the MadScientist Margue, is about to make sure that she'll become fully independent and self-aware. This ultimately causes Sharon, until then a harmless VirtualIdol, to become an UnseenPenPal who both inherits and twists Myung's love for Isamu, going {{Yandere}} for him and becoming the LoveMakesYouEvil-motivated BigBad.]]

* In ''Anime/MagicalGirlLyricalNanohaAs'', 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 at the hospital. 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. [[FromBadToWorse Things quickly get worse from there.]]

* ''Literature/SwordArtOnline'': Shouzou Yuuki's decision to [[ArrangedMarriage engage]] Asuna to [[BitchInSheepsClothing Nobuyuki Sugou]] and make him the head of RECT Progress results in 300 SAO survivors being trapped and forced to endure inhumane mind control experiments, and Asuna being trapped in ALO for months and nearly being raped. As a result, RECT nearly goes bankrupt and Shouzou's reputation is ruined when Sugou's cyber-crimes are exposed to the public.

*** ...Though according to the CompilationMovie series, [[InSpiteOfANail maybe not]]. In this version, [[spoiler: Jeremiah actually notices Shirley the moment she gets her memories back and warns her not to get involved any further for her own safety. This results in her encounter with Rolo being avoided, but Lelouch raids the camp anyway due to how dangerous the aforementioned experiments still are, which was always his other reason.]] The events mostly proceed as they originally did, with a couple of other changes such as [[spoiler: Shirley still being SparedByTheAdaptation, the Black Knights being more reluctant and not as quick to judge Lelouch (Ohgi and Viletta's roles above essentially downplayed to being just as confused and unsure as everyone else and wanting to ask him some questions first, with Schneizel's group being the ones trying to gun him down before they could get their answers), and ultimately a SequelHook that leads into the ''Lelouch of the Resurrection'' film.]]

* In Creator/RiyokoIkeda's other manga, ''Manga/{{Claudine}}'', there's [[spoiler: Claudine's best friend Rosemarie.]] Depressed because [[spoiler:Claudine doesn't return her feelings ''and'' she has found out that Claudine's crush Cecilia is actually the lover of Claudine's *father* Auguste]], she mentions this very complicated deal to [[spoiler: her teacher Louis, who is also Cecilia's younger brother.]] Little did she know that [[spoiler:Louis ''also'' had been Auguste's lover in the past... and that he would go {{Yandere}} on Auguste and Cecilia.]] The result isn't pretty: [[spoiler: Auguste and Cecilia die, the maddened Louis disappears, the Montesse family is devastated, and Rosemarie herself [[TwoFaced gets half her face horribly burned]].]]

** One of One For All’s bearers’ Nana Shimura, ends up setting a lot of things into motion when she decides to give up her son, Kotaro, into foster care in an effort to protect him from All For One after her husband was murdered. This left Kotaro carrying a massive grudge against his mother, thinking she chose her heroic career over raising him, that carried over into his adulthood, resulting in him becoming a abuser towards his children whenever they brought up heroics, with his son, Tenko, getting the brunt of it due to his own wishes to be a hero. [[spoiler: One night, while locked out of the house as punishment, he ends up awakening his Decay Quirk, destroying his house and inadvertently killing his dog, sister, mother and grandparents. And when Kotaro finally sees the scene and Tenko approaches him begging for help, the man’s response was to whack him with a weed clipper, resulting in his death, Tenko’s sanity being destroyed and him wandering the streets while All For One found him, eventually growing up to become Tomura Shigaraki. When the truth comes out, all Nana can do is weep in despair, knowing what her decision to abandon Kotaro had caused.]]

* In ''Manga/JoJosBizarreAdventure'', George Joestar I's well-intentioned adoption of Dio Brando leads to his death, his son's death, his grandson's death, his great-great-granddaughter's illness, his great-grandson's (temporary) death, his great-great-great-grandson's death, his great-great-great-great-granddaughter's death, the deaths of countless others, and nearly causes the end of the ''entire universe''.

** On a less cosmic scale, a trio of schoolgirls are the reason why a serial killer was able to get away for so long. To explain: [[spoiler:these girls encountered a kid who was mortally wounded by said killer. Rather than go get help or call a teacher, they instead leave him. This in turn causes him to be finished off by the killer, and thus, allows him to get away scot-free for at least a few more arcs]].

* In ''Anime/SailorMoon'': Sailors Uranus and Neptune believe that victory cannot be attained without some [[MartyrWithoutACause horrible angsty sacrifice that only they have the moral strength to make]]. So basically, they make such choices that would make things worse at the end of the run. What they did in the GrandFinale is a major example in their habit to make bad decisions: [[spoiler:in a desperate gambit to reach for Galaxia themselves and keep their beloved Moon from dying, they pull a FakeDefector act that includes [[KillTheCutie killing Pluto and Saturn]] (depriving Moon of two valuable allies ''right after the death of Princess Kakyuu and the {{Heroic Sacrifice}}s of the Inners''), attacking Moon herself and her remaining allies the Starlights (so she wouldn't suspect them, but also [[CurbStompBattle weakening her physically]] [[BreakTheCutie and devastating her emotionally]]) and ultimately attack Galaxia on their own using powers she gave them after they faked choosing to serve her. But they didn't count on [[NoSell Galaxia being immune to the attack]] and they end up dying in front of Moon, Chibi-Chibi Moon and the Starlights [[SenselessSacrifice without accomplishing anything in the end]].]]

* ''Anime/FateZero'', being a prequel to ''VisualNovel/FateStayNight'', has some individuals who make seemingly innocuous decisions that have consequences later.

** Also during the D-Reaper saga, it’s revealed that [[spoiler: Jeri’s father unwittingly caused a lot of Jeri’s issues which the D-Reaper took advantage of. On the day of her mother’s death, he stated that her death “was fate”, [[note]] Only in the Japanese version [[/note]] inadvertently traumatizing her further and sowing the seeds for her issues in regards to fate/destiny. And while he did try his best to raise Jeri, he ended up becoming somewhat cold and stern towards her (with a particular example being him refusing to go pick her after she came back from the Digital World since she had left on her own), resulting in him not really noticing Jeri’s issues or realizing just how much she was hurting in the inside. When he’s finally forced to confront his screw ups when Jeri is abducted and used as a power source for the D-Reaper, he is very understandably left guilt stricken and tries to save her himself, but ultimately couldn’t do anything for her]].

** Also during the D-Reaper saga, it’s revealed that [[spoiler: Jeri’s father unwittingly caused a lot of Jeri’s issues which the D-Reaper took advantage of. On the day of her mother’s death, he stated that her death “was fate”, [[note]] Only in the Japanese version [[/note]], inadvertently traumatizing her further and sowing the seeds for her issues in regards to fate/destiny. And while he did try his best to raise Jeri, he ended up becoming somewhat cold and stern towards her (with a particular example being him refusing to go pick her after she came back from the Digital World since she had left on her own), resulting in him not really noticing Jeri’s issues or realizing just how much she was hurting in the inside. When he’s finally forced to confront his screw ups when Jeri is abducted and used as a power source for the D-Reaper, he is very understandably left guilt stricken, crying out for Jeri]].

* ''LightNovel/GoblinSlayer'': In the past, one female adventurer spared one goblin child when it supposedly promised to be good, however not only did sparing said child turn out to be a bad idea when it killed her the moment she turned her back, said goblin child eventually grew up to be a goblin lord that tried to attack the farm that Goblin Slayer himself lived at.

* In ''LightNovel/HighSchoolDXD'', Azazel is indirectly responsible for Akeno Himejima's estrangement with her father Baraqiel after Shuri Himejima's murder.

* ''LightNovel/RebuildWorld'': TheArtfulDodger Alna is one when she pickpockets 100,000 aurum from the protagonist Akira and gets away with it by manipulating TheRival Katsuya into protecting her. What this ends up resulting in, is a massive MobWar fought by the two biggest gangs in the slums, who are each an NGOSuperpower, after [[spoiler:said pickpocket is used as TheBait along with the kidnapped Sheryl in a scheme exploiting Akira's [[BerserkButton Berserk Buttons]] to get him to attack one of said gangs and have the other take advantage, in a GovernmentConspiracy to be rid of both gangs cheaply.]]

* In the [[AllThereInTheManual notes]] left by the late ''Anime/TrinityBlood'' author, Sunao Yoshida, Seth Nightroad's actions led to two damning consequences. First, it's revealed that she injected the Crusnik nanomachines on her oldest brother, Cain, to save him after he got into an fatal accident during the Red Mars colonization project. Unfornately, it made Cain insane and drove him to destroy the world. Though Cain secretly hates humanity, the nanomachines that Seth injected just gave him the power to do so which resulted to killing Lilith Sahl and taking her nanomachines. It wasn't clear if Seth knew what she had done but she realized that Cain is no longer the brother that she and Abel loved. Second, according to the notes, [[spoiler:she activated the satellite cannon of the Ark hidden in the imperial palace to destroy nuclear military base of the Vatican to prevent Francesco from using it when the Terran-Methulesah war broke out again and this gave Cain the chance to use the Ark and have Dietrich fire the satellite cannon on Rome, resulting to the Vatican's destruction. This bites back her in the dust ''badly'' when she stopped Cain from using the Ark which got her killed, her nanomachines stolen by him and the Empire going for an all-out counterattack on the humans by conquering their territories]].
* ''LightNovel/SwordArtOnline'': Shouzou Yuuki's decision to [[ArrangedMarriage engage]] Asuna to [[BitchInSheepsClothing Nobuyuki Sugou]] and make him the head of RECT Progress results in 300 SAO survivors being trapped and forced to endure inhumane mind control experiments, and Asuna being trapped in ALO for months and nearly being raped. As a result, RECT nearly goes bankrupt and Shouzou's reputation is ruined when Sugou's cyber-crimes are exposed to the public.

* ''LightNovel/UndefeatedBahamutChronicle'': [[spoiler:Celestia Ralgris revealed the Empire's corruption to Lux's grandfather Wade Roadbelt, who confronted the emperor about it. In response, the emperor imprisons Wade and leaves him to die and banishes Lux's side of the family from the court, creating a chain of events that leads to the death of his mother, the fall of Arcadian Empire, and him being reduced to a local chore boy. Unlike most examples of this trope, Celestia is fully aware of what her actions have caused and has felt guilt over it for the next five years.]]

*** Remember how Majin Buu became truly evil, killed off half of the Z-warriors, the entire human race, and then finally blew up Earth? It all happened because of a few goons in the episode "The Evil of Men", where they attack and shoot Bee (Buu's pet puppy) and Hercule (Buu's new best friend who was in the process of rehabilitating him). As a consequence Buu snapped and steamed out [[EnemyWithout Evil Buu]], who then [[SplitPersonalityTakeover proceeded to absorb him]]; the entire rest of the Buu saga is the ''entire universe'' paying the price for what those thugs did. They themselves faced consequences a bit sooner -- Evil Buu vaporized one with a ki blast, and Super Buu killed the other by liquifying himself, entering the guy's body, and causing him to inflate until he burst.

* ChildSoldier Falco Grice in ''Manga/AttackOnTitan'' falls victim to this trope. Being a kind person, he notices one of the returning [[ShellShockedVeteran Shell-Shocked Veterans]], who is missing AnArmAndALeg, is wearing his [[ANaziByAnyOtherName ethnicity-identifying armband]] on the wrong shoulder and fixes it for him. Falco then begins visiting the man at the mental hospital and ends up posting letters for him, as the man says the government intercepts any mail sent by patients there. It turns out that the man is [[spoiler:Eren Yeager]] who is there to [[spoiler:sneak into the Rebellio district of Marley]] and meet up again with [[spoiler:Reiner Braun]]. During a local festival, which is attended by many politicians, military officials, and nobility, Falco [[spoiler:takes Reiner to Eren at Eren's request and claim that they were [[BlatantLies old friends]]). While Eren and [[VillainousBreakdown Reiner]] are talking, Falco eventually works out that Reiner would've met Eren during his mission on Paradis Island, and that Eren is the enemy. Falco calls Eren out on being used, but Eren doesn't seem fazed by it. After peace talks between Eren and Reiner break down, Eren turns into his Titan form and attacks the festival, killing a major Eldian named Willy Tybur, who is giving a speech about how [[AntiHero evil]] Eren is and how he plans to destroy the world]]. It's currently unknown how many people in attendance ended up dying, but Falco's actions caused them, and the deaths of [[spoiler:his fellow ChildSoldier friends Zofia and Udo]], as well.