History UnwittingInstigatorOfDoom / Literature

** Rhaegar Targaryen's abduction on Lyanna Stark set the ball rolling on a chain of events that ultimately brought about both of their deaths, the fall of the Targaryens from power, and a long generation enmity among several Houses who were affected by the war.

* In the former trope-naming incident, from the Literature/SherlockHolmes story ''The Final Problem'', a Swiss messenger in Moriarty's employ catches up to Holmes and Watson with a message saying that an Englishwoman at the hotel they're staying at is on the verge of death and wants to see an English doctor. Watson obediently returns to the hotel, unknowingly [[spoiler:leaving Holmes to face Moriarty alone at the edge of Reichenbach Falls, apparently leading to both of them falling to their death]]. However, it is implied that [[spoiler:Holmes knew all along what was going down but saw no reason to put Watson at further risk.]]

** Supreme Chancellor of the Republic Tarsus Valorum in the [[Literature/DarthBane Darth Bane trilogy]] ended up doing this. After the New Sith Wars ended with the Battle of Rusaan, he passes the Ruusan Reformations, which among other things [[PacifismBackfire disbands the Republic's military and also forces the Jedi to take a more advisory role]] in TheRepublic rather than the protectors that they used to be. This would later play into Palpatine's hands when he took advantage of Jedi Master Sifo-Dyas's contracted clone army to create an army [[MindControlDevice unquestionably loyal to him]], rather than thinking men that might form their own opinions. He facilitated the rise of TheEmpire a thousand years later, though he couldn't have known it at the time.

** A Jedi Padawan by the name of Leor Hal managed to both save and doom the galaxy. He was a Jedi Padawan who around 132 BBY came up with the theory of Potentium, a philosophy which believed that [[BadPowersGoodPeople you could use whatever Force powers you wanted and the Dark Side wouldn't affect you if you simply refused to let it]]. Yoda and the rest of the Jedi High Council branded him TheHeretic and expelled him & his followers from TheOrder. His teachings directly led to the [[Literature/LegacyOfTheForce "Unifying Force" theory and the Second Galactic]] CivilWar, however after leaving TheOrder he discovered the RoguePlanet Zonama Sekot which was instumental in ending the [[ScaryDogmaticAliens Vong]] War.

* ''Franchise/BuffyTheVampireSlayer'': In ''Bad Bargain'', amateur warlock Michael Czajak and his mother unwittingly cause the main conflict of the book. Mrs. Czajak thoughtlessly donates a medallion she finds to the school rummage sale without asking Michael, who uses it for protection against the demons that plague Sunnydale. Desperate fo retrieve his medallion, Michael casts a summoning spell in the school basement, not realizing he is standing near the Hellmouth portal, which weakens whenever magic is cast nearby. This causes everyone in the school to be infected by demonic diseases, and eight people die.

* Jacob in ''Literature/TheRedTent'': Because he demands an outrageous bride price for his daughter (after a [[DoesNotUnderstandSarcasm sarcastic]] comment by Joseph), and a combination of greed and overprotectiveness, his sons murder Dinah's TrueLove, and all the other Shechemite men. Jacob [[WhatTheHellHero calls his sons out on it]] and dies [[MyGodWhatHaveIDone full of regrets]].

* ''Literature/TheWitcher'': Ciri accidentally [[spoiler:introduces ''the bubonic plague'' to The Continent after a diseased flea hitches a ride on her jacket during a space/time jump from Medieval Europe.]]

** Regulus Black in the {{Backstory}} revealed in ''[[Literature/HarryPotterAndTheDeathlyHallows Deathly Hallows]]''. Fresh out of school, he joined the Death Eaters . He had a change of heart and tried to defect [[spoiler:after he found out Voldemort's biggest secret, that he had split his soul into [[SoulJar horcruxes]]. He didn't tell anyone else he knew to be against Voldemort (ie his brother or Dumbledore) and decided to destroy it himself. He wasn't able to do it but had created a fake to put in its original location. This delays the eventual finding of the others by almost two decades (since Dumbledore had suspected there were more but didn't have any sort of proof), drags out Voldemort's downfall, and causes countless deaths]].

* ''Literature/ThebeAndTheAngryRedEye'' combines this trope with FirstContactFauxPas. A BenevolentAbomination in the form of a giant, snakelike alien lives inside UsefulNotes/{{Jupiter}}. When the heroes' ship approaches the planet, the being attempts to say hello with a radiation surge. Tragically, the surge hits the ship, causing a series of disasters that winds up killing all but one of the astronauts.

* According to ''LightNovel/DanganronpaTriggerHappyHavocIF'', Chihiro Fujisaki. [[spoiler:Using his skills as the [[TechnoWizard Ultimate Programmer]], he created the software for a new robot. That robot's name? ''Monokuma''. Yes, '''''[[HangingJudge Monokuma]]''''']].

* Jacob in ''Literature/TheRedTent'' Because he demands an outrageous bride price for his daughter (after a [[DoesNotUnderstandSarcasm sarcastic]] comment by Joseph), because of a combination of greed and OverprotectiveDad factor, his sons murder Dinah's TrueLove, and all the other Shechemite men. Jacob [[WhatTheHellHero calls his sons out on it]] and dies [[MyGodWhatHaveIDone full of regrets]].

** Supreme Chancellor of the Republic Tarsus Valorum in the [[Literature/DarthBane Darth Bane trilogy]] ended up doing this. After the New Sith Wars ended with the Battle of Rusaan, he passes the Ruusan Reformations, which among other things disbands the Republic's military and also forces the Jedi to take a more advisory role in the Republic rather than the protectors that they used to be. This would later play into Palpatine's hands when he took advantage of Jedi Master Sifo-Dyas's contracted clone army to create an army unquestionably loyal to him, rather than thinking men that might form their own opinions. He facilitated the rise of the Empire a thousand years later, though he couldn't have known it at the time.

** Crown Prince Duncan Targaryen who abdicated his birthright to the Iron Throne to MarryForLove Jenny of Oldstones led to the Mad King Aerys taking the throne and to the fall of the Targaryen Dynasty.

** Remus Lupin in ''[[Literature/HarryPotterAndThePrisonerOfAzkaban Prisoner of Azkaban]]''. [[spoiler: If he'd remembered to take his potion before the climactic confrontation, he wouldn't have been a threat when he became a werewolf, and could have continued to concentrate on keeping Pettigrew from getting away. And if Pettigrew hadn't gotten away, and had been turned in to the authorities, Sirius would have been cleared of his murder charges and would no longer have to be a fugitive, meaning he could have a closer relationship with Harry. And if he'd had a closer relationship with Harry, Voldemort wouldn't be able to use their separation to trick Harry, and Sirius wouldn't have ended up dead. Furthermore, if Pettigrew hadn't gotten away, he wouldn't have been able to play a pivotal role in expediting Voldemort's return to a fully-formed body and enable the tragedies of books 4-7 -- including Lupin's own demise at the hands of the Death Eaters.]]