For Want of a Nail



""What happens if Bobo causes one of those weird numbers where he goes back into the past and steps on a butterfly or something, and because of that mammals as we know them never evolve, and because of that mankind never invents slot machines, and my favorite hobby goes straight down the toity, huh?!""

- Pearl, Mystery Science Theater 3000

An Alternate Timeline or Elseworld (often in time travel stories) in which one small change has a ripple effect, resulting in massive changes. The technical term (from one of the seminal papers on chaos theory) is "sensitive dependence on initial conditions", but it's better known colloquially as "the butterfly effect". It's a Wonderful Plot is a subtrope where the protagonist is the nail. Butterfly of Doom is a subtrope where no good deed goes unpunished. The Unwitting Instigator of Doom is the character who steps on the butterfly/distracts the hero at the wrong moment/innocently reveals the Big Secret and will be hated for it.

Derives from a proverb-turned-poem which traces its origins as far back as the 14th century (making this Older Than Print). Probably the most well-known version is the one which appeared in Benjamin Franklin's "Poor Richard's Almanac": "For want of a nail, the shoe was lost; For want of a shoe, the horse was lost; For want of a horse, the rider was lost; For want of a rider, the message was lost; For want of the message, the battle was lost; For want of a battle, the kingdom was lost, And all for the want of a horseshoe nail."

Said poem is itself derived from a verse by George Herbert's Jacula Prudentium, written in 1651.

A Deal with the Devil will often have the "nail" as a price, something of seemingly small consequence, that is in fact huge.

A Sub-Trope of What If, and often a Sub-Trope of Make Wrong What Once Went Right. A Super-Trope of "Close Enough" Timeline.

Compare All the Myriad Ways. If the originals meet their alternates, may result in Other Me Annoys Me or Future Me Scares Me.

Often overlaps with In Spite of a Nail, because many stories wouldn't be that interesting if everything was different.

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"For want of a paperclip, a page was lost. For want of a page, a contract was lost. For want of a contract, a deal was lost. For want of a deal, a factory was lost. For want of a factory, a company was lost. All for the want of a paperclip."
 * A Staples Business Depot TV commercial from the 90s, directly references the trope with a different scenario.


 * Toshiba Electronics commercials depict this inside the mind of the CEO, leading him to hold back on releasing the product to make sure it has all the features it needs, regardless of the cost. For example, adding shock-proof hard drives to their PC's. He imagines an electrician dropping his laptop, picking it up and plugging it into an outlet at a relay substation, which causes a spark that knocks out the power. This leads to a person's roommate opening the refrigerator, sniffing some milk, noticing it's spoiled but drinks it anyway, immediately turns into a zombie, and unleashes the Zombie Apocalypse on the world.

Anime & Manga

 * Higurashi no Naku Koro ni has a bad case of this; every arc it's something else in the initial factors--minor things like a man moving into the village or even Keiichi giving someone a doll--that sets off a completely different scenario of bloody tragedy.
 * The doll was important to the one receiving it. That's why it changes things.
 * The basis for the plot of Dual Parallel Trouble Adventure is that reality splits depending on whether a construction worker on site when Lost Technology is found takes a sample home when his crew's ordered to destroy the evidence. The Hero, Kazuki is at first able to see into the other reality, and eventually crosses over.
 * The reboot of the Rozen Maiden manga features Jun, the protagonist of the original manga, who decided to take the blue pill path instead, resulting in him not meeting the dolls and remaining a Hikikomori for the remainder of his mid school duration. We meet him as a disgruntled college student with a bad side job, and frequently being laughed at due to his past status as a hikikomori. That is, until Shinku and the other dolls from the original continuity appear in his continuity...
 * One of the recurring themes in Legend of the Galactic Heroes, where seemingly minor events cause a huge impact on history. What if Kircheis had been armed during Ansbach's assassination attempt ? What if the supply depot Muller attacked had fought back instead of surrendering, so that he couldn't rescue Reinhard? What if Hilde hadn't persuaded Mittermeyer to attack Heinessen to force a ceasefire, or if Yang ignored said ceasefire? What if Yang hadn't allowed the "Imperial escorts" on his ship ?
 * In the Mai-Otome manga, Sergay is not Nina's father and Nagi, while not a very good person, is not the main antagonist. As a result, Nina stays on the heroes' side for the entire story.
 * On a lesser scale, Sergay is not anonymously paying for Arika's education in the manga, forcing her to take on several side jobs and leaving her no time to study, at one point putting her in danger of failing and dropping out.
 * In Dragonball Z, the main timeline ends up being VERY different than the one Trunks comes from for reasons that aren't really well explained. For example, Androids #17 and #18 are much more powerful in the main timeline but not really all that evil (they do initially intend to kill Goku out of boredom but don't really go out of their way to hurt anyone else). Android #19 didn't exist in the Trunks' timeline nor did Android #20 (Dr. Gero had never turned himself into an Android). Goku also got sick MUCH later than he did in Trunks' timeline.
 * Actually much of this was explained. 17 and 18 were altered to be more powerful because of Trunks's arrival and the power increases of the other characters. Since Goku died in Trunks's timeline he saw no need and spent time making the androids into world domination machines (adjusting their personalities for the task) still strong enough to take on inexperienced Super Saiyans but still weaker than the main timeline versions. Gero probably altered himself and created Android 19 because of the appearance of more combatants with formidable power levels. This was because Trunks warned them about the androids and they trained to become more powerful. Also, in Trunks's timeline Goku was the one who fought Frieza on Earth by teleporting using instant transmission and the fight caused the virus to take affect when it did. Since he didn't go through any big fight, when Trunks killed Frieza, Goku never caused the virus's progress to accelerate and kill him when it did in Trunks's time.
 * The difference in strength between the main and alternate timeline versions of #17 & #18 was explained differently in the manga. In the ‘Trunks: The Story’ chapter when Gohan was fighting #17 & #18, immediately before #17 kills Gohan, #17 tells him that he’d been using no more than half of his power. After a short timeskip we see Bulma & Trunks debating the need for a time machine. Trunks thinks he’s already gotten strong enough to defeat 17 & 18, while Bulma thinks he’s too naïve since he was only at about Gohan’s level and Gohan was still killed. After this we see Trunks leave to fight them and the next panel we see they’ve put him in the hospital. There is no actual proof that the full magnitude of 17’s power (and possibly also 18’s) was revealed to Trunks before he went back in time. Assuming that Trunks was unaware of 17 & 18’s full power in his timeline, it would explain his surprise at their power in the main timeline. This would also indicate that the androids from the main timeline were actually just as strong as those in the alternate timelines and that Trunks was simply unaware of that fact.
 * In Blood +,  are twin sisters. Joel raised   like a daughter, whereas Amshel kept   locked in a tower, treating her like an experiment. When   receives this history lesson, she can't help but sympathize with her sister, knowing full well that they could just as easily wound up with their roles switched.

Comic Books

 * The Elseworlds comic, "Justice League: The Nail", where Jonathan and Martha Kent's truck is incapacitated by, you guessed it, a nail (stuck in the tire). This prevents them from going into town, which prevents them from seeing Superman's rocketship crash land, which means they never adopt baby Kal El (he is instead ), which means Superman doesn't become the DC universe's iconic hero. Without the Big Blue Boyscout around to inspire confidence and good feelings about metahumans among the general populace, the American people (particularly those in Metropolis) become highly suspicious of all superbeings, even to the point of hunting them down and locking them up simply for existing. The story even begins with the above poem.
 * It turns out that
 * And also The Joker  It's not even shown on-panel but it's still Nightmare Fuel.
 * And in the end, the cycle closes with
 * Another well-known Elseworlds comic, Superman Red Son has a slight difference in the rotation of the Earth as the "nail", so Superman lands in the Soviet Union, eventually rising to become leader of the union. Similarly, Superman: True Brit... you get the idea.
 * Star Wars did its own spin on this with the limited comic series Star Wars: Infinities, each story exploring how such a change would affect each of the Original Trilogy films. In specific...
 * In A New Hope, Luke fails to destroy the Death Star because one of his torpedoes malfunctions. As a result, the Empire catches several Rebels trying to escape Yavin, including Leia, who Vader trains as his apprentice. In the meantime Luke trains with Yoda, and after five years he confronts Vader and Leia, ultimately ending when Yoda Colony Drops the Death Star on top of the Emperor's palace.
 * In The Empire Strikes Back, Luke dies on Hoth, leading Han and Leia to seek Yoda. Lando doesn't betray Han, getting Cloud City destroyed. C 3 PO ends up in Vader's hands and finds out about Dagobah, leading him to confront Yoda (and get into a mental battle against the spirits of Jedi passed like Qui-Gon and Mace). Vader gets killed by a surprise attack by Han, leaving him and Leia to rejoin the Rebels and fight the Emperor.
 * In Return of the Jedi, C 3 PO gets disabled by accident, forcing Leia to reveal her identity to Jabba and allowing Boba Fett to escape with Han in the confusion. Yoda dies before Luke can hear his final message, meaning Luke (and the Emperor) sense his death and Luke gets captured leaving Dagobah. Meanwhile, Leia mounts a rescue mission to save Han, but he's become permanently blind. Because the heroes don't befriend the Ewoks, they attack both sides at the shield generator, meaning Han and Lando have to take it out. Luke and Leia manage to redeem and rescue Vader, who dons a white costume and joins the good guys in hunting down the Emperor.
 * In a nice touch, the highly trained Force users can sense that things are not as they should be, though they don't quite know what is going on. They just know the Force is going nuts.
 * Star Trek has the series The Last Generation, where the assassination plot from Star Trek VI the Undiscovered Country succeeded, resulting in the Klingons dominating the galaxy as far as the Next Generation era, with most of the TNG cast as scarred resistance fighters, Worf as the leader of the Klingon Empire, and Sulu using the Excelsior to make hit-and-run raids on the Klingons as "the Silver Ghost".
 * The Prophecy of the Destruction of the Green Lantern Corps foretold that all of the enemies would unite to destroy the Corps. The turning point would be when the Five Incursions of the Empire of Tears would join to destroy the Corps, Ranx destroying Mogo with a Blink Bomb and the Children of the White Lobe killing Sodom Yat. All the players except the Empire of Tears joined in the Sinestro Corps War, all because Sinestro pissed the Kingdom of Tears off back during his days with the Green Lantern Corps. When still a good guy, Sinestro arrested the leader of the Kingdom of Tears for their part in murdering Abin Sur, whose replacement Hal Jordan would lead the Green Lantern Corps into battle to save the Corps.
 * Of course, this also ties into the War of the Lights/Blackest Night prophecy; at the start of the Sinestro Corps War, the Guardians revolted against Ganthet, the Guardian who insisted on recording the prophecy of how the Green Lanterns would be destroyed and who sought to use the prophecy as a guidebook to try and stop Sinestro and his allies. They kicked Ganthet out, which led to him forming the Blue Lantern Corps and pretty much began micromanaging the Green Lantern Corps to such an extent with petty rules to weed out emotions, that it has led to the Corps being weakened due to mass resignations, most of which have been orchestrated by a mole who's leading the Black Lanterns to wipe out the Green Lanterns.
 * In Watchmen, Jon Osterman's flashbacks in Chapter 5 reveal that he believes that Janey Slater's watch band breaking is the nail that lead to the
 * The watch band breaking led to Osterman becoming Dr. Manhattan, and thus to the differences between the story's history of the Vietnam War and Cold War and Real Life's history, but the timeline had diverged before then during World War II or so when costumed vigilantes began operating, and there's no definite "nail" for that part. (Although the presence of costumed heroes as reality meant that superhero comics were all but dead. The big thing? Pirates.)
 * Note here that Osterman probably wouldn't acknowledge there was a "nail" at all. Since he sees all times in his life simultaneously, his worldview is that everything in his life is fated to happen - that is, there is no divergent timeline that the "lack of a nail" could create.
 * Marvel Comics has the What If? series, a title based entirely on this trope. Issues that occur include what would have happened if Spider-Man had joined the Fantastic Four; what would have happened if Rick Jones had been turned into the Hulk instead of Bruce Banner; what would have happened if the Invisible Woman had stayed with Namor; what would have happened if Jane Foster had found Mjolnir instead of Dr. Donald Blake; what would have happened if Aunt May had been killed by the burglar instead of Uncle Ben, etc. What's especially noteworthy about these series is how much worse off many of the characters tend to be if things didn't turn out the way they did (interestingly, Spider-Man, usually Marvel's whipping boy, comes off better than before at least as often as he comes off worse, probably because the default state of his life is suuuuuck).
 * One particularly noteworthy example is the story Bullet Points, where one bullet kills Dr. Erskine and Ben Parker. Effect?  All because of one bullet!
 * While What If generally used this trope straight, stories occasionally played with it. One story has the Fantastic Four never receiving their powers, but becoming entirely successful adventurers anyway.
 * And one of the more interesting playarounds in this series is an issue in which Dr. Doom never suffers the accident that injured him and twisted him into a supervillain. Rather, he becomes a superhero, and life actually goes from strength to strength for him. On the other hand, once again, it is his own ego that causes the driving complication of his life.
 * Exiles can be considered a Spiritual Successor to the What If? series; the team would frequently visit realities where one small thing had massively changed how things had happened, and there would always be at least one page explaining what the 'nail' was and its various knock-on effects. Generally, but not always, their job was to repair the reality by setting things back to how they should have been in that reality. This does not always mean changing reality to be like the standard marvel universe but instead to fix things so they are the way that universe was originally supposed to turn out. The general idea was that something caused a number of the different realities to be messed up and the main characters were traveling around fixing them so that they were back on track, even if that track was not the same as the standard marvel universe.
 * One slightly silly issue showed the team being the nail for a change, preventing an alien invasion by buying the last bagel in a bakery store.

Fan Fiction

 * Ace Attorney Fanfic A Complete Turnabout demonstrates how easily this can happen in a universe, which's characters only have One Degree of Separation between each other. Simply, removing the DL-6 incident from the first game radically alters the history of the entire cast. Among other changes, Phoenix is now an amoral prosecutor, while Edgeworth, never having been taken in by von Karma, is a defense attorney.
 * A popular concept is to rewrite the Harry Potter books with Harry "having common sense" but they change so much (and his common sense is really having unexplained knowledge of what's happening next) that the actual fic barely counts.
 * One of the few that actually manages to succeed is called Run That by Me Again, and that's only because it doesn't take itself too seriously, and it's a drabble serious instead of an actual rewriting.
 * There is an entire subset of people who like to write Alternate History stories about our own world.
 * The Buffy the Vampire Slayer fan-fic "Difficult to Fight Against Anger" is a What If story where Warren Mears doesn't shoot Buffy and kill Tara. The timeline diverges when Buffy, Xander, and Willow decide to go the sidewalk cafe to talk about the past year's events, and Warren, finding only Tara there, forces his way in and rapes her.
 * Hell, Buffy and Angel have plenty of these. From Faith not trying to ignore the fact that she killed a person and therefore not becoming evil (it's a common thing if Fuffy stories) to a different relationship changing things (oddly enough, this tends to be Fuffy or Wuffy), the fandom like this. Alot.
 * The "Horseshoe Nails" series by Dyce is named after this trope. These River/Jayne friendship fics gradually become romance, and the 'Verse is changed at least somewhat for the better. In this case, the "nail" would be the first fic in the series, Two Gorram Days.
 * The Nail in the first chapter of Misfiled Dreams is Rumisiel putting Ash's and Emily's files back in place.
 * Shinji and Warhammer 40 K: Shinji finds a chest with titular game in it - and becomes a Crazy Awesome Magnificent Bastard.
 * All He Ever Wanted starts off with the UK maintaining the alliance with Japan in the 1920s. Stuff happens. But given its nature, the POD is just the only visible difference from history and Hetalia canon.
 * There is actually a GinRan Bleachfic titled "All for the Want of a Nail", with each chapter referencing a line from the poem. While the chapters in question are ridiculously short, the nail in question is Gin wanting to stay back in Soul Society to raise the kid he's suddenly having with Rangiku.
 * Harry Potter and the Sage's Stone: The description of the fic is the For Want of a Nail poem. It starts off with one Ebenezar McCoy ending up in Godric's Hollow the night Voldemort, and salvages young Harry from the place before Dumbledore arrives.
 * The Star Wars Infinities above are just one set of possibilities. Star Wars fandom can go absolutely bonkers with these, as even the slightest thing being different could have huge, galactic-spanning consequences.
 * Luminosity is entirely based around this. In a Romance Novel where the point is that the plot we see revolves around Bella, what happens if Bella is smart and introspective? Despite the plot being all-but unrecognizable, that is the only change until the story starts.
 * Halo: the Art of War: In the original Halo canon, a Covenant probe hitches a ride on a UNSC ship and finds the UNSC stronghold planet Reach, then uses a Slipspace jump to get back to its masters. Master Chief and some of the other SPARTANs were about to go on a classified mission when the Covvies attacked, and most of them were killed. The first game takes place right after the UNSC Pillar of Autumn has fled the battle and found Halo. This fic imagines what would have happened if the probe had never made it back and the mission was allowed to commence unmolested. All the cool guns, however, are just Author Appeal and Rule of Cool.
 * Team 8. The indirect nail is Kurenai getting a Dark and Troubled Past that has (so far) only been alluded to. The more direct one is driven in during the first chapter: Kurenai personally asks for Naruto Uzumaki for her genin team (sending Kiba over to Team 7 with Sasuke and Sakura), resulting in, among other things, a much more mature, focused and introspective Naruto.
 * A popular topic among fans of The Legend of Spyro is what if Cynder's egg had been saved by Ignitus instead of Spyro's. What isn't clear is whether this makes things better or worse off.
 * Played for Laughs in the Harry Potter fanfic ''A Bad Week at the Wizengamot". It starts off with the Wizengamot finding Harry guilty of underage use of magic. By the end of the first chapter, the Ministry of Magic has collapsed, 99% of all Ministry officials have been arrested by the European Magical Union, and Sirius Black has been appointed Interim Minister of Magic.
 * Rurouni Kenshii uses this concept with the nail being that the main character is female instead of male. Instead of meeting Tomoe, she meets Enishi. This causes Enishi to go from a crazed Big Bad to Kenshii's partner. Had the fanfic gotten far enough, chances are the last manga arc either wouldn't have happened or it would have been on a far smaller scale (lacking a mastermind and a budget).
 * Ultimate Spider-Woman: Change With The Light uses this trope in that
 * The Back to The Future fanfic The Almanac Chronicles has several, but one was very significant to the history of the Alternate Timeline.
 * The Pony POV Series has two seperate timelines, the Epilogue timeline, a Villain World ruled by Discord, and the main timeline, where Discord lost just as he did in the series. The only visable cause of the split between the timeline is that in the Epilogue timeline, Twilight Sparkle broke in the maze while she broke slightly later in the real one. A piece of Recursive Fanfiction writen explorers how this worked.
 * Backward with a purpose is a time travel fic in which Harry, Ron and Ginny goes back into time and use their knowledge of the war to prevent the deaths of their family and the eventual cost of the war that resulted in over 40% of the wizard society’s death. Harry, Ron and Ginny are Genre Savvy enough to change only certain things to keep the timeline changes minimal, but the combination of bad luck, another time traveler and the fact that they built stronger relationships with certain minor characters resulted in unforeseen changes to the timeline. Fair warning for anyone who wants to read this, it has some sexual scenes.
 * Also, the nail in question was never mentioned but happened sometime in the 7th year of the original timeline, in which for some reason the horcrux hunt took longer than it should have, resulting in a higher casualty rate.
 * The Wrath of the Half Blood Prince, a Harry Potter fanfic where Death Eater wannabe Mulciber pressed Severus Snape's Berserk Button by targeting Lily Evans instead of Mary Macdonald for a prank which got a throwaway mention during Snape's PensiveFlashback in DeathlyHallows.
 * In the Game of Thrones / A Song of Ice and Fire fic "The Joinery", the entire premise is whatif Ned Stark became king instead of Robert Baratheon and married Cersei Lannister, which changes the plot significantly, but not enough to save Ned.

Film

 * Lola rennt (a.k.a. Run Lola Run): the wildly variant endings of Lola's twenty-minute Groundhog Day Loop can all be traced back to Lola's reaction to the boy with the dog on the staircase.
 * Sliding Doors has catching/not catching a train as the "nail" and ends with dramatically different versions of the protagonist.
 * The 'nail' moment was marked with music and later in the film when the protagonist had to make a decision the music was played again. Although we didn't follow this 'split' it implied that there were more points where a single decision would be important.
 * However, the event that happened in one case at the beginning of the film still happened in the other case at the end of the film, after many near misses, so on a broader timescale the system was not that sensitive to initial conditions.
 * Some events are constant between the two timelines. The system is still highly sensitive, though:
 * In the movie Frequency, a son manages to send a message back in time and save his firefighter father's life. However, because of this change, his mother, who was a nurse, never left the hospital to make funeral arrangements and so was on duty to save the life of a man who should have died. This man turns out to be a serial killer who kills the mother and is now still at large in the present of the son.
 * The whole "Pottersville" sequence in It's a Wonderful Life is an example of this. George's having never been born turns out to have an indelible effect on the lives of pretty much everyone he's ever known in real life.
 * The Back to The Future film trilogy is a series of nails showing what would have happened if a certain event did or didn't occur:
 * In the first movie, Marty McFly's father was a wimp who couldn't stand up to the bully Biff Tannen, but after Marty changed events concerning his father in 1955, he becomes a more confident man who has Biff eating out of his hand, and Marty's family life has also improved for the better.
 * In the second movie, Grey's Sports Almanac was the nail that, when old Biff from 2015 gave it young Biff in 1955, caused Hill Valley in 1985 to become Biff's lawless empire where he killed Marty's father and married Marty's mother, adopting her children in the process. With the nail removed, history reverted to how it originally unfolded in the first movie.
 * Also in the second movie, an automobile accident that would have happened in the third movie when Marty was racing with his friend Needles was the nail that caused Marty's future history to go down the toilet in 2015, eventually resulting in his being fired from the company he and Needles were working for. When the accident was avoided in the third movie, the YOU'RE FIRED message that Marty's girlfriend/future wife Jennifer Parker had received from the future was erased, meaning that their future could potentially become a better one.
 * Lampshaded by the scriptwriters in the Audio Commentary of the first Pirates of the Caribbean movie. The entire plot is basically kicked off by a corset being laced too tightly.
 * The Narada being sent back through time causes a divergence, creating the 2009 Star Trek reboot. James Kirk's father dies by suicide-charging his ship into it, James himself is born in space rather than Iowa, he and Spock do not meet on the best of terms,
 * Don't forget Starfleet technology being more advanced thanks to the scans of the Narada taken by the Kelvin. Then there's the destruction of an entire Klingon fleet, as well as a dozen newest Starfleet ships with.
 * Rules Lawyer and borderline Jerkass Genie Rumpelstiltskin offers Shrek a Deal with the Devil in Shrek Forever After: Shrek gets one day as a lawless ogre in exchange for one day from his past. Rumpelstiltskin takes the day Shrek was born, thus creating a world in which Shrek never existed. One of the biggest changes is the fate of Fiona, who without anyone to save her from the tower chose to Screw Destiny and simply escaped by herself.

Literature
"Homer: Eh, "Close Enough" Timeline."
 * The Draka: In the book Drakas! S.M. Stirling uses the poem in the introduction., , , , , , For Want of a Nail, Spell My Name with an "S"
 * Malcolm Gladwell writes about this trope, where something unknown becomes a major factor in society., , ,
 * Shakespeare Did It First (sort of): "A horse! A horse! My kingdom for a horse!" Although Shakespeare doesn't dwell on it (Alternate History not having been invented yet), it certainly indicates the thought that England would have been rather different had Richard won the day at Bosworth Field.
 * In Under A Velvet Cloak, Jolie is attempting to defy this trope.
 * The children's story Why Mosquitoes Buzz in People's Ears: A West African Tale a mosquito lying leads to a chain of events where the sun does not rise.
 * A Sound of Thunder, by Ray Bradbury, in which stepping on an actual prehistoric butterfly changes modern politics for the worse (see In Spite of a Nail, though).
 * The Simpsons did an homage/parody of Bradbury's story in "Treehouse of Horror V". The short "Time and Punishment" had Homer sent back in time to the prehistoric era, where the mere act of crushing a mosquito resulted in creating a future where Ned Flanders was an omnipotent dictator. Homer's repeated trips to the past to rectify this only make things even more bizarre. (There's one where things seem to be beyond perfect...but there's no such things as donuts. Apparently; immediately after he departs to the past, donuts start falling from the sky, and Marge remarks that it's raining again.) In the end, Homer ended up with everything normal, except that humans were now born with prehensile forked tongues.

"Ridcully: I suppose we'd have settled down, had children, grandchildren, that sort of thing ... Granny: What about the fire? Ridcully: What fire? Granny: Swept through our house just after we were married. Killed us both."
 * Another version: William Tenn's 1948 short story, "Brooklyn Project". The "acting secretary to the executive assistant on public relations" describes a government time-travel experiment to a group of journalists, explaining that some scientists were foolishly concerned that a probe sent into the past might by its very presence inadvertently change the present. But this is a ridiculous notion, of course: the story ends with the journalists dissolving themselves into liquid and flowing up to examine the time travel apparatus, while the acting secretary extends his fifteen purple blobs and exclaims, "Nothing has changed!"
 * Another example is the eponymous novel, For Want of a Nail by Robert Sobel, in which a British victory at Saratoga leads to a very different North America. Guess where the title comes from.
 * The Animorphs series does this twice in the Megamorphs books. In "Elfangor's Secret', Visser Four radically alters history by changing things in the past. "Back to Before" has Crayak giving the kids a chance to not go through the construction site. Things are radically changed and they actually beat the Yeerks much faster than in the regular timeline, but at the cost of a large number of other lives in the process, (ok more lives than in the regular timeline)
 * SF writer Harry Turtledove is very fond of this concept. A recent short story of his featured in a mid-2007 edition of Asimov's Science Fiction magazine examined what World War II politics - up to and including the war itself - may have been like if the press had been less supportive and more suspicious, all started by a single news report basically accusing the administration of causing the bombing of Pearl Harbor through its own incompetence. However, he's most well known for a series of books featuring an entire alternate history of the United States, where the premise is that a Confederate messenger picked up a cigar he dropped. As a result, one of military history's great strokes of luck - the interception of a Robert E. Lee's complete plans for troop movements, which were wrapped around said cigar - never happened, and the whole tide of The American Civil War is turned as a result.
 * McClellan's most defining state in the Civil War was a massive exaggeration of the forces arrayed against him. Had he not captured the Confederate plans, which detailed the strength of the Confederate forces, he would have believed that he was massively outnumbered. (Prior to finding the orders, he thought that Lee's portion of the army held more soldiers than the entire combined forces of the CSA - in the entire war.)
 * This is also a running theme in his works--as the books stretch out and the timeline diverges from our own, characters will either succeed or fail at tasks by small margins, leading them to question what their lives would have been like had the event in question not happened.
 * In Turtledove's Atlantis series, the American Civil War is replaced with a massive slave revolt that happens partially because a slave tripped over a loose floorboard.
 * In The End of Eternity by Isaac Asimov, the time police exploit this professionally and with mathematical precision. The book begins with the main character changing the course of history by moving a small object onto a different shelf.
 * Also by Asimov (he liked this trope) is Spell My Name with an "S", where two aliens bet as to whether they could avert a nuclear war on Earth with a minor stimulus. Sure enough, getting one person to change how they spell their name works.
 * In the Discworld novel Jingo, Vimes' Dis-Organiser reports the events of a Alternate Universe in which Vimes decided keeping the peace in Ankh-Morpork was more important than leaving the city to chase after his suspect. The result was that the entire Watch (except Colon and Nobby) died in battle before Vetinari could complete his plan to stop the war.
 * Also spoofed in The Last Continent, when the wizards realize that they've been sent back in time, Ponder warns Ridcully of the potential ramifications: "The obvious thing to worry about is killing your own grandfather." To which Ridcully simply replies, "I can't see why I'd do that. I rather liked the chap."
 * In a different book, Rincewind remarks that killing your own grandfather is the only thing that really appeals to him in this whole time travel business.
 * Ridcully then goes on to point out to Ponder that stepping on insects is fine, since history-as-they-know-it either isn't affected by the deaths, or actively requires them.
 * This trope is played with in Night Watch-
 * Deconstructed/Averted Trope in Lords and Ladies:

"Rebecca: And your father met your mother in this wise. Some thirty-six years ago, as he was walking down Regent Street, his attentions were directed to a sculptor's shop, in which was a remarkable monument to a Colonel Culpepper, who died of a cold caught in going into the Ganges to rescue a favourite dog which had fallen into it. An old schoolfellow passed by, and, touching your father on the shoulder, asked him to dinner. Your father went, and at the dinner met your mother, whom he eventually married. And that's how you came about. [...] If your father hadn't loitered opposite the sculptor's shop, his schoolfellow would never have met him. If Colonel Culpepper hadn't died, your father would never have stopped to look at his monument. If Colonel Culpepper's favourite dog had never tumbled into the Ganges, the Colonel would never have caught the cold that led to his death. If that favourite dog's father had never met that favourite dog's mother that favourite dog would never have been born, neither would you. And yet you're proud of your origin!"
 * The Trope Namer is referenced in Thief of Time, where there is mention of a battle which was won and the kingdom saved because Lu Tze "just happened" to be walking by the side of the road with a portable forge.
 * He manages a reverse of this in Small Gods by improperly cooling a key piece of a war machine. Making sure For Want of a Nail situations turn out as they should is a big part what History Monks do.
 * Might be a double subversion as the war was meant to happen, since that was how history of supposed to go. Lu Tze, however, decided differently...
 * Subverted in Mort, where the titular character spares the life of a princess who was due for assassination. Perhaps because the History Monks overlook this one, the bubble of Alternate Timeline this creates around Princess Keli (in which she's alive) gradually shrinks around her, as Destiny (in which she died on schedule) re-asserts itself. Note that having her still be alive seems to have altered the weather in her vicinity, probably as a Shout-Out to the Butterfly Effect.
 * In The Science of Discworld III, the wizards have to time travel around our earth making very minor changes to prevent Darwin from being killed/never being born
 * Two Nightside novels by Simon R. Green steadily establish that is responsible for everything that has ever gone wrong in the Nightside, dating back to its very creation.
 * To Say Nothing of the Dog discusses the trope quite a bit. It has an interesting scenario involving the destruction of Coventry Cathedral. Because of cracking the Enigma Code, British intelligence officials knew in advance that it would be bombed. The novel imagines a scenario where the MacGuffin, a piece of hideous Victorian architecture, disappears during the air raid and an old busybody deduces it was stolen just before the air raid by someone with prior knowledge of the raid,, and sends a letter about this oddity to the newspaper. When Nazi intelligence officials come across this letter, they deduce  that their code has been broken and adopt a new one which the Allies can't crack, and thus win World War II. The removal of the MacGuffin means that 'time' has to 'fix' things by eliminating the old busybody. The method? Send the heroes back in time to ensure she is happily married and nowhere near Coventry Cathedral.
 * And all of that? Is to
 * W. S. Gilbert (of Gilbert and Sullivan)'s 1881 play Foggerty's Fairy may be an archetypal example; the protagonist makes a supernatural deal to alter his past so that he never met a certain woman. Dire consequences follow, and subsequent acts take place in alternate universes as he frantically tries to fix his past. The trope is exemplified by the words of the Fairy Rebecca:

"Ursault the All-Seeing: Of course, if your boy Dammet remembers to tie the brindle dog when the harvest moon blooms full, the white maid will never be. A lot of trouble that will save. [...] But on the other hand, a lot of trouble that will cause. This same wolfwere maid could bring doom to the floating city. A lot of trouble that will save. On the other hand..."
 * In the ingenious novel Time and Again by Jack Finney, the hero deliberately cuts himself off from the present by . Why doesn't this set up a paradox? It isn't that kind of book.
 * The short story "Wikihistory" by Desmond Warzel has this as the reasoning behind Hitler's Time Travel Exemption Act: "no Hitler means no Third Reich, no World War II, no rocketry programs, no electronics, no computers, no time travel. Get the picture?" Hong Xiuquan, however, has no such protection.
 * In the short story A Little Knowledge by Elaine Cunningham in Realms of Shadow diviner cannot produce any useful prophecy because there's just too many drastic changes invoked by small causes:

"Think of what might have been. Dragon fire and savage swords in Eriador, night in Rivendell. There might be no Queen in Gondor. We might now hope to return from the victory here only to ruin and ash. But that has been averted - becuase I met Thorin Oakenshield one evening on the edge of spring in Bree. A chance-meeting, as we say in Middle-earth."
 * Cruelly subverted in the end:
 * Subverted in Jasper Fforde's The Eyre Affair. Thursday Next's time-traveling father asks her what happened in two important battles. In both cases the commanders were killed by an incredibly lucky shot just as the battle started. Thursday points out that this couldn't possibly be the work of French revisionists as, in both cases, the second in command took over and won the battle anyway. Her father points out that he never claimed that the revisionists were any good at it.
 * The Eric Flint novel The Rivers of War posits an Alternate History whose point of departure is caused by Ensign Sam Houston not taking an arrow between the goalposts in the Battle of Horseshoe Bend, unlike what happened in Real Life. Instead, he just gets nicked by the arrow on his outer thigh, thanks to slipping when coming over a dirt mound.
 * Only person-wide case: A brief disruption in a baby "factory" in Brave New World means that forgetting one injection on an embryo would cause him to be "the first death by trypanosomiasis for over half a century".
 * The premise of His Dark Materials is the Many-Worlds theory (quantum superpositions create parallel universes), and that one can travel between these worlds. Depending on when and where this split happens, this has more or less drastic effects on the world. We have: our universe; one where the Church has absolute power and souls live outside the body; one where, apparently, Britain is a Mediterranean country; one where two dust-influenced lifeforms evolved; one devoid of life; The Land of the Dead; and one where the dominant physiology is vastly different. And that's just those they visited. Good thing they never opened a portal to a universe without Earth...
 * The Trope Namer is referenced in the second book of Madeleine L'Engle's Time Quartet series, A Wind in the Door, when Calvin O'Keefe explains to Mr. Jenkins the reason the Ecthroi are trying to eliminate a seemingly insignificant person, Charles Wallace Murry.
 * All of the events of the Codex Alera can be traced back to a servant girl in a rural valley wanting some flowers, by way of some coincidences and a Nice Job Breaking It, Hero moment. Otherwise, there would have been no Horde of Alien Locusts, the protagonist wouldn't have met the First Lord and gotten a scholarship to the Academy, and Lord Aquitaine's plan to become First Lord would have gone off without a hitch, ending a centuries-old dynasty but otherwise keeping the country's status quo pretty much exactly the same. Because of the flowers, though,.
 * Before that, everything was caused because Without that, there wouldn't have been a succession crisis in the first place.
 * Arabian Nights tale, "What a drop of honey caused" is based off this: A hunter brings a find of honey to a merchant, but a drop falls onto the ground, which attracts insects, which attract birds, which attracts the merchant's cat, who attracts the hunter's dog who kills the cat. The merchant kicks the dog out of anger and the dog dies. The hunter kills the merchant out of anger/revenge. The merchant and hunter were from neighboring villages who fight, but the villages are either side of the border between neighboring provinces, and tit-for-tat exchanges start one of the bloodiest wars in local history, all because of a drop of honey.
 * In Stephen King's book Needful Things, Leland Gaunt, the proprietor of the titular shop, exchanges small treasures for ridiculously low prices if the buyer agrees to do him a small, mostly inconsequential favor. The favors inevitably snowball into large-scale disasters,
 * King also does this with ''11/22/63", where
 * Possibly subverted in Philip Roth's book The Plot Against America, in which the Republican nomination of Charles A. Lingbergh for President in the 1940 Election keeps the US out of the war a good while longer, and massively increases antisemitism. At the end of the day, however, the Allies only win a year later than they did in real life; it's the damage to the Jewish diaspora in America, and the solidarity of the American people as a whole, that's the major outcome.
 * This concept is the driving force behind the events in the book 'A Crack In The Line'. The book is about a boy whose mother is dead and who lives with his father and his mother's older sister. He accidentally finds a way to travel to an alternate timeline, where a female version of himself exists; her mother is still alive and she has no aunt. They discover that their two universes exist because of an event in the past that wasn't suppose to happen, causing the timeline to split off in two. Initially, they believe the divergence happened when their mother got in the accident which in the boy's world, killed her. However, the actual divergence happened when.
 * In The Never War, the third installment of The Pendragon Adventure, Bobby goes to Third Earth (Earth in the early 51st Century AD) to find out what would happen if he saved the Hindenburg. Turns out, if said Zeppelin was saved, London, DC, and New York would've been nuked by the Luftwaffe just before D Day, and things would've gone down mountain from there.
 * In the appendices of The Lord of the Rings, Gandalf says how things might have been, if Smaug the dragon had not been killed in The Hobbit:


 * Alfred Bester's The Push of a Finger is built around this trope: a future-predicting machine reveals that the Universe will be destroyed in one thousand years unless the protagonists find and avert the single event that'll put everything in motion.
 * Mary Lavin's short story "The Story of the Widow's Son" is based around this, examining the different consequences of whether the titular boy did or did not accidentally run over an old hen with his bike.

Live-Action TV
"Martin: You know, I don't like this red bowl, it's throwing everything off!"
 * Frasier. Sliding Frasiers is an episode based on Sliding Doors, in which two paths of Frasier's life are examined on whether he chose to wear a suit or a sweater for a speed dating service.
 * Another example is when we're shown Martin and Daphne's extraordinarily efficient morning routine on several occasions, but on the last one Daphne puts Martin's cereal in a red bowl instead of a yellow one. The whole routine goes to hell, culminating in Martin accidentally throwing his toast on the floor.

"Martha: But are we safe? I mean, can we move around and stuff? The Doctor: Of course we can. Why do you ask? Martha: It's like in the films. You step on a butterfly, you change the future of the human race. The Doctor: I'll tell you what, then -- don't step on any butterflies. What have butterflies ever done to you?"
 * Star Trek the Original Series, "City on the Edge of Forever"
 * Specifically, the event here is.
 * Star Trek the Next Generation, "Yesterday's Enterprise"
 * Also "Tapestry": Q shows Picard that by correcting one mistake in his youth, a seemingly beneficial act, he never becomes captain of the Enterprise. (He specified, however, just to assuage Picard's conscience, that he's "not that important" and that no one would die or otherwise be terribly affected by this choice except for himself.)
 * The final season episode "Parallels" has Worf jumping from parallel universe to parallel universe, where this trope is in full effect. La Forge is dead in about half of them, Picard is dead in about a quarter of them, Wesley Crusher's a lieutenant in a few of them, and the last gasp effort to set things right is almost disrupted by an Enterprise where the Borg have pretty much conquered everything. Worf retains his memories of everything, however, which inspires him to try to romance Troi.
 * One of these universes appears to have the Bajorans throwing off the Cardassian occupation and proceeding to become as bad as the Cardassians.
 * Star Trek Enterprise. The entire Temporal Cold War appears to be based on this trope. In "Shockwave" Captain Archer is due to be interrogated by Big Bad Silik, so he's yanked off the turbolift by time agent Daniels and taken to the 31st century Earth--which is now in ruins because of Daniels' action. In "Storm Front" Nazi Germany is winning the Second World War, apparently because a temporal agent assassinated Lenin. And in "Twilight" the fact that Archer is disabled by a Negative Space Wedgie means he's unable to save the human race from being wiped out by the Xindi.
 * Star Trek Voyager has the "Year of Hell" two-parter involving a Krenim timeship making subtle and not-so-subtle changes to the timeline hoping to create a perfect timeline where their empire is once again powerful and all their loved ones are alive. One part featured Chakotay offering to erase an insignificant-looking comet from history, thus preventing the Voyager's interference in Krenim affairs. The Krenim captain explains that Chakotay would be wiping out half the species in the sector (a little hypocritical, given what the captain is doing) due to this comet being involved in seeding most of the inhabited planets in the sector billions of years ago. (As the timeship itself exists out of time, destroying it at any point causes it to never have been built, and leading to a more or less happy ending for everyone involved.)
 * The Voyager novel Echoes occurs when a planet activates a revolutionary new transport system that happens to shift the residents over one universe. When the Voyager is inadvertently summoned by the energy pulse, it is immune to the shifts. Residents report small changes in the world around them as they're moved. This wouldn't be such a problem, except somewhere down the line, the planet was hit by a meteor. That universe's Voyager was tasked with trying to save a few billion people. And a few hours after that, a few billion more. And a few hours after that...
 * The episode "Non Sequitur" shows what would have happened if Harry Kim was not chosen to be among those who would be in Voyager's crew, with the results also affecting the life of Tom Paris. Of course, the catch is that this is an alternate reality in which Harry Kim still remembers being a crew member of Voyager and has somehow wound up in this reality.
 * This was the most common plot in the first few seasons of Sliders, with such universes as "Exactly like ours, but the atom bomb was never invented," "Exactly like ours but antibiotics were never invented," "Exactly like ours but one of the heroes was Elvis," etc. This plot became less common as the series progressed.
 * This is directly referenced in one episode, wherein the device that creates the wormholes initially cannot be fixed because in this parallel Earth everyone had an almost superstitious aversion to higher technology.
 * In Red Dwarf, it is revealed that the pathetic and cowardly Rimmer diverged from his ludicrously cool alternate-universe counterpart Ace at one critical juncture: one of them was held back a grade in school, and the other was not.
 * More noteworthy, it is implied that every Rimmer in every dimension has the full unbridled potential to become Ace, provided they get the training and the wig.
 * Lampshade Hanging in the Doctor Who episode "The Shakespeare Code":

"Narrator: "A chain of critical events that led to the X disaster...""
 * Also in Doctor Who: this trope is the central premise of the series 4 episode "Turn Left", exploring all the horrible things that would have happened if Donna had taken a different job three years ago. For example, Also,
 * Previously done in "Father's Day" when Rose saves her father from being killed by a hit-and-run driver. In keeping with the combination of For Want of a Nail and In Spite of a Nail that Doctor Who leaps between periodically, saving one man in the past leads to...flying dragon demon things trying ot unmake reality.
 * However, this is explained in that at that moment, there were a pair of Doctors and a pair of Roses, making it, "...a vulnerable point."
 * Buffy the Vampire Slayer's Wishverse is the result of this trope combined with a subversion of Wonderful Life. Because Buffy never came to Sunnydale, Giles is no longer a watcher, Willow (now a skanky Lesbian Vampire) and Xander were turned into vampires, Angel became a prisoner implied to be tortured and raped for the fun of the vamps, and The Master left his underground prison, wreaking merry havoc on the town. Buffy also got herself a scar and an unrefreshing new attitude. The Wishverse debuted in "The Wish", and our favorite Lesbian Vampire crossed over into the Buffy Verse in "Doppelgangland" for even more fun.
 * Although this may seem a bit less dramatic as the trope would imply. Considering that Buffy was the one to kill the Master and keep the Hellmouth in check, it's not a dramatic leap in logic for her lack of presence having disastrous consequences, unlike a kingdom falling for want of a nail.
 * This happens again in the Angel episode "Birthday", where Cordelia turning her head the other way at a party results in a world where Cordelia has become a famous TV star instead of joining up with Angel, but Wesley's lost his arm and Angel, having to take the visions in place of her, has lost his sanity without her presence.
 * Oddly enough, this alternate-Angelverse implies that Angel and Doyle kissed. No, really.
 * To clarify, before, Doyle kissed Cordelia to give her his vision power. However, since Cordelia never joined the group, Angel had to receive the visions instead, Wesley (the only other permanent member of the group in season 1) not being around at the time. Hence...
 * The Stargate SG-1 two-parter "Moebius" shows how, by meddling with time, SG-1 created an alternate timeline in which the Stargate was never found in Giza, and how it affected (or, rather, ruined) their lives -- that is, until the people of that universe set off to Set Right What Once Went Wrong.
 * The SG-1 direct-to-DVD movie Continuum had a similar setup (except Ba'al caused it intentionally, and the Stargate was found but promptly lost again), with plenty of resulting changes. Unusually, some of it was quite positive--for instance, Jack O'Neill's son, dead in the original timeline, was doing just fine. The interesting thing here is that Jack's son died in a tragic accident which bore no relation to the Stargate program at all. So the butterfly effect must have been at work in really subtle and far-reaching ways...
 * The Stargate was lost when the ship carrying it across the ocean to America was lost with all hands on board. One effect of one of those people dying was made explicit in the film:
 * The Scrubs episode "My Butterfly", where two possible days are shown based on whether the eponymous butterfly lands on a woman's cleavage or a fat guy's... well, cleavage. Everything that goes wrong in the first version goes right in the second,
 * Scrubs also utilizes this with an 8 year "feud" between J.D. and the Janitor being the result of a penny someone dropped in a sliding door.
 * Malcolm in the Middle did an episode which shows two versions of the family's trip to the local bowling alley, with one parent taking the older kids while the other stays home with Dewey. Ultimately neither version is better or worse than the other.
 * The Heroes episode "The Butterfly Effect" hinges around a future character's presence in the present horribly messing up the future, with things getting worse and worse the longer he stays and tries to repair things.
 * Friends had an episode's opening end with the cast asking themselves how different their lives would have been if they had taken different paths in life, e.g., Ross' wife never leaving him, Chandler quitting his job and becoming a columnist, Monica not losing weight etc. This episode and the next are called "The One That Could Have Been."
 * In Kamen Rider Den-O, Big Bad Kai uses this in an attempt to prove to Ryotaro that he's right to try and change history. Sending an Imagin back in time, he successfully kills as a teenager, resulting in a reality that seems to be happier overall.  The series' method of handling Time Travel soon restores the dead character to life, and Kai's attempt to sway Ryotaro fails.
 * This is a major plot point of the upcoming Lets Go! Kamen Rider movie. Eiji and Ankh follow New Den-O into the past and Ankh drops one of his Cell Medals. Shocker finds the Medal and reverse engineers it, leading to a Bad Future where Shocker was able to defeat the original two Kamen Riders and conquered the world.
 * In the 100th episode of Charmed, a largely empowered Cole wished Paige had never reconstituted the Power of Three with Piper and Phoebe. Paige crosses over to the new reality (as she is teleporting at the time of the change) and has to witness the changes in her sisters' lives: Phoebe and Cole have an unhappy open marriage, Piper became a rogue demon hunter obsessed with getting revenge for Prue's death and she and Leo are divorced, the Manor is inhabited by demons with Daryll working for them, and P3 is in ruins.
 * Another episode had the sisters Mental Time Travel to the future, when the knowledge of the supernatural is made public, and witches are being hunted and burned. Why? Because one of them used a spell on a guy they didn't like, who proceeds to have a burning hate for all witches. When they go back to the present, they undo the event, although we're not shown that future again.
 * In the Season 2 Supernatural episode "What Is and What Shall Never Be," like in the Buffy example above, this trope combines with a subverted Wonderful Life so that Dean wishes that the fire that killed Sam and Dean's mother never happened. She and Sam's girlfriend, Jessica, are still alive, but Dean's a horrible asshole and he and Sam have a terrible relationship, and the Wonderful Life part comes in when Dean finds out that . And then.
 * In Season 6 an angel goes back in time and saves the Titanic from sinking. This resulted in thousands more people being alive today and many changes but the brothers are immune to this effect for the most part and their own lives are not significantly changed.
 * In season 6 of Lost, the successful detonation of a bomb on the island in 1977 causes an alternate timeline where the plane never crashed; smaller details (also probably resulting from the bomb detonation) include Hurley having nothing but good luck and some people, such as Shannon, not being on the plane.
 * Now it appears that the bomb didn't cause it, since certain changes (such as Ben and Roger leaving the Island) may have occurred earlier. So we know that there's a nail wanting, but not what the nail is.
 * Unfortunately, this is all now Jossed by the revelation that the alternate timeline !
 * Or it worked but,
 * In Farscape, the crew go on a planet that commemorates/celebrates a peace between Peacekeepers and the race on that world, which at that time was barbaric/tribal. Stark accidentally sends them back to those events, and each of their actions is seen to have at least local effect on the planet: from the changes (rather stagnation) of the local language, to many deaths and even the total destruction of the planet while the crew from the past make more and more effects, and Pilot is horrified. They manage to pull a last-minute all-out old-fashioned killing, which actually saves the planet and puts the planet exactly to where it was...except the women and children who were instrumental in the peace process died, the memorial lamenting their senseless deaths and the peace that followed because of shame over them.
 * In other episodes, it's suggested that Crichton even thinking something different in a wormhole will create an alternate reality/timeline. Even the closest reality almost gets his father killed.
 * In the Season One finale of Primeval, while Nick and Helen are in the distant past, they accidentally let the offspring of a future predator run wild. When they return, they notice slight changes in the timeline that seem completely unrelated to that one change. Their tiny unnamed department in the Home Office has been turned into a full-fledged Anomaly Research Center. Also, Claudia Brown turns into Jennie Lewis, who, initially, has a completely different personality from Claudia, but later becomes more and more like her.
 * As of the season 4 premiere of Eureka, this trope is in full force. Lampshaded when Carter points out Kevin is no longer autistic as omething that logiclly shouldn't have changed and Henry says that no one knows what causes it in the first place.
 * Interestingly, a character later tries to fix something that happened in the 40s due to him being absent at the time. However, he realizes that this particular event can't be changed.
 * An episode of Quantum Leap had Sam leap into a younger Al, trying to save him from being executed for a crime he didn't commit. As the probability of the younger Al's execution approaches 100%, the older Al is replaced, but when Sam discovers a vital clue, Al is restored, and Sam completes the mission with the help of both Als. The nail?
 * An earlier episode had the Quantum Leap Project about to have its federal funding cut off. When Sam helps a woman out in the past, however, she ends up replacing the committee chairman in the present that was about to cut off the funding, and renews it instead.
 * One could say that Sam is leaping to provide nails throughout the history of his lifetime.
 * The nail comes about when Sam is resting on a bed with the woman. Turns out she is studying for the bar exam, so he offers to quiz her. But the nail turns out to be . Interestingly, Al seems to notice while he was in court; and the shift occurs right when the decision was being read.
 * Smallville did this with its tenth season episode, "Luthor", in which Clark visits Earth-2, a world where he was found by Corrupt Corporate Executive and Diabolical Mastermind Lionel Luthor, instead of the Kents. In it, Lionel has literally taken over the world, using Clark as his muscle and Lex and Lutessa as the brains; he's far more evil than his Earth-1 counterpart, having had ultimate power thrust into his hands. Clark himself is Ultraman, a homicidal maniac and Psycho for Hire who serves as his "father's" Dragon. Lutessa (Earth-1 Tess Mercer) is acknowledged by her father, is living with him, and is sleeping with her adopted brother Clark. Lex is dead, having betrayed the family and tried to murder Ultraman. Lois Lane and Oliver Queen are engaged; Oliver is hated for having evicted most of the town of Smallville, just so he can mine for Kryptonite. The Justice League does not exist, and even Oliver isn't Green Arrow, just an angry millionaire with a self-esteem problem. All because Superman became a villain.
 * There is also that episode when Brainiac and Kara traveled to the past. Clark was aware of this, but he did not care since he thought everyone's life would be better if Brainic succeeded in killing him. Cue Jor-El giving him a sight of how the world would be then. Lex is the President, Kara was found and raised as a Luthor, Brainiac is Lex advisor, all the Meteor Freaks have been killed, and Brainiac soon will release Zod in a soon to be destroyed world.
 * Head of the Class discussed this when Mr. Moore brought up how the world (specifically America and Cuba) would be different if Fidel Castro was a better baseball player, and went pro instead of becoming President of Cuba. Alan thought it silly, and compared it to Ronald Reagan staying an actor instead of going into politics, which Mr. Moore thought was an excellent point.
 * Fringe: In "The Plateau," a man with artificially increased intelligence uses this concept to kill people. For example, putting a pen on top of a mailbox causes a chain of events that leads to a woman getting run over by a bus.
 * A later episode, "The Firefly," is pretty much an analysis of this trope.
 * The entire Community episode "Remedial Chaos Theory" is about this: at Troy and Abed's housewarming, nobody wants to go downstairs to get the pizza, so Jeff rolls a six-sided die to decide who goes. This proceeds to create six separate timelines where different things happen (the darkest one includes Pierce getting accidentally shot and dying, Jeff losing an arm in a fire, Annie suffering a mental breakdown, Shirley becoming drunk, Troy losing his larynx, and Britta dying a strip of hair blue) until the real timeline.
 * The documentary series Seconds Before Disaster is centered on this trope and lampshades this a lot.

"Mainwaring:For want of a nail, the shoe was lost; for want of a shoe, the horse was lost; for want of a horse, the rider was lost; for want of a rider, the message was lost; for want of the message, the battle was lost; and so it was that the kingdom was lost -- Jones: Hence the expression, "Keep your hair on.""
 * Air Crash Investigation also has episodes that explain about air crashes caused by this trope. See "Real Life".
 * Dads Army features Captain Mainwaring (attempting to be philosophical) reciting part of the relevant poem, only for Lance-Corporal Jones to interject at the end:

Music
"For want of a bird The sky was lost For want of a nail A shoe was lost For want of a life A life was lost For want of a toy A child was lost"
 * The verses to "It's All Who You Know" by the Newsboys are two scenarios of this trope.
 * In the first verse, a slip-up in an actor's facelift leads to the failure of the show that would have saved a TV network.
 * In the second verse, an dogsled racer forgets to take a cough drop and loses his voice. He and his dogs freeze to death when "the sled got snowbound."
 * Tom Wait's song "Misery Is the River of the World" even includes a line from the trope namer.


 * The video for "Has It Come To This?" by The Streets features a split-screen of two possible day-in-the-life situations of Mike Skinner.
 * "The Want of a Nail" by Todd Rundgren.
 * The whole point of Jay-Z's American Gangster is basically showing the mirror universe where Sean Carter kept on selling drugs instead of going into music.
 * Bowling for Soup's "Almost"'s verses each have chain of events that might have happened, if not for the first event in the chain only "almost" happening:
 * Verse 1: "I almost got drunk at school at 14 / and I almost made out with the Homecoming Queen / who almost went on to be Miss Texas / but lost to a slut with much bigger breastes / who almost dropped out to move to L.A. / and was almost famous for almost a day"
 * Verse 2: "I almost held up a grocery store / where I almost did five years and then seven more / 'cause I almost caught for a fight with a thug / who almost made off with a bunch of the drugs / that I almost got hooked on when you ran away / and I wish I could've had the nerve to ask you to stay"
 * Verse 3: "And I almost wrote a song about you today / but I tore it all up / and then I threw it away"

Mythology

 * In Norse Mythology, Freyr is doomed to lose the fight to Surtur at Ragnarok because he gave his good sword away to Skirnir.

Play By Post Games

 * Fate Nuovo Guerra has at least two so far, being an Alternate Universe story and all. More obviously, the destruction of Fuyuki City caused the Einzberns to pack up and move operations to Lucca, where the next Holy Grail War takes place. Less obviously, the lack of a living Shirou results in the very existence of the Edelfelt Sisters Lumi and Lempi.

Tabletop Games

 * The current default setting for GURPS is based on mixing this idea with The Multiverse to produce an infinite number of worlds. There's at least one canon parody of the concept with the world "Enigma" in which the only difference is the band The Clash never formed (and then the planet all humans on the planet disappeared without a trace in 1982).
 * During Magic: The Gathering's Time Spiral block, the middle set (representing the present) featured "alternate universe" versions of certain cards, primarily reprinting older cards into other colors that would best represent them if various mechanics hadn't been assigned as they were. Keeping in the spirit of "What if?", the flavor text on a good portion of the cards give quick looks at an entire subplot that revolved around what would've happened if Mirri had slain Selenia before Crovax had a chance to, thus fulfilling the terms of a family curse, turning her into a vampire instead of him.
 * In a nice scenario for a Dr Who style RPG the player's TARDIS comes to rest in the car park of a film studio. This causes John Wayne to have to find another spot so he is late and angry for his meeting to discuss playing Genghis Khan. Instead Ronald Reagan gets the role and cancer inducing radiation dose. President Wayne's foreign policy isn't a great success either.
 * John Wayne was a very heavy smoker throughout his life (at one point up to six packs of cigarettes a day!) and in 1964 nearly died of lung cancer (one lung was removed and The Duke needed supplemental oxygen for the rest of his life). Any contamination he might have received from the set of The Conquerer was likely just a Wafer-Thin Mint given what he'd already done to himself.

Video Games

 * The Legacy of Kain games deal with the attempts of the immortal vampire Kain to change his fate and that of the world of Nosgoth, which are tied. Frequent use of time travel is made by Kain, Raziel, and Mobius the Timestreamer to advance their own agendas. For example: Mobius traveled back in time in the first chapter of the series "Blood Omen" and corrupted the King William the Just, turning him into The Nemesis, who threatened to conquer all of Nosgoth in the future. Kain inadvertently travels back to this time period, where he kills William the Just before he manages to become The Nemesis. When he returns to the present, Kain realizes that by killing William the Just he has sparked a genocide against vampires that left him the last in all of Nosgoth.
 * One of the drama CDs based on Guilty Gear revolves around an Alternate Universe in which the war lasted ten years longer because Ky Kiske was killed in the penultimate battle, and in which the Gears would have won if I-No hadn't gone back in time to rescue him.
 * Chrono Cross features the as its nail. This seemingly meaningless event changes entire worlds, though, as, for example, one character that's dirt-poor in one world is filthy rich in another. His family still resents him in either timeline though.
 * Technically, he resents his family. And that's not the strangest difference, either. Consider the twin sisters that were never together. Ever. In fact, they shouldn't even be twins! Huh?!
 * The nail turns out to be, the previous example is really just a side-effect.
 * The two spinoff games to Scarface, The World Is Yours and Money. Power. Respect., are based on Tony escaping the film-ending shootout, while the comic Scarred for Life is based on Tony surviving wounds that were fatal in said shootout. Tony's survival either way is a small but very important difference, with above scenarios being impossible "normally".
 * The original Mega Man series and four of its spinoffs have focused on Dr. Light's aptitude for robotics and the battles his creations have fought in the process of trying to keep a rival from taking over the world. However, Dr. Light focusing on internet technology instead of robotics results in the Alternate Universe Battle Network and Star Force spinoffs. Wily is still a roboticist, however; his actions start out as revenge against society for pushing the field of robotics aside.
 * Chrono Trigger features a scene where Marle, sent into the past, is mistaken for the missing queen Leene; they call off the search, meaning that the real queen stays missing and is presumably killed. This has the unfortunate effect of causing her present-day descendant, Marle, to cease to exist. The part is notable because later timeline changes don't suffer the same "paradox correction"; for instance, you can raid the Black Omen in one era, then travel back and raid it in an earlier era, and still keep the spoils from the previous raids (though if you raid it in one era and then try again in a later era, everything will already be gone). Apparently they had different writers with different ideas.
 * Word of God says that the team hadn't worked out the rules for time travel until after they'd painted themselves into a corner with that plot point.
 * In Resistance: Fall of Man, Europe comes off prosperous and peaceful after WWI, preventing the Second World War. However, instead of a Revolution, aliens invade Siberia. Russia can't catch a break, can it?
 * This is a major plot point in Dark Chronicle. Emperor Griffon has destroyed the "orgin point" of several important places, which Max and Monica have to fix.
 * The Fallout series revolves around this trope: slight changes in the direction of politics and technology after the end of WWII resulted in a completely different world just a few decades after, and ended turning the world into an apocalyptic wasteland.
 * Indeed, in Fallout 2 the player can go through the time-portal from The City on the Edge of Forever from the original Star Trek. The player ends up in (spoilers) Vault 13. If he taps a computer, it sets up a water chip to fail, which will end up causing...
 * The entire premise behind Command and Conquer: Red Alert.
 * And then happens again in Red Alert 3. Apparently, no one thought that.
 * Discussed in Magical Girl Lyrical Nanoha A's Portable: The Battle of Aces, where it is suggested that Fate would have turned out like the Ax Crazy Berserker Material-L if she had never met Nanoha or the others. Nanoha disagrees with this suggestion though, noting that Fate is a stronger person than that, and that with Arf, she is not truly alone.
 * Duncan serves as the nail in Dragon Age and its various Origin stories; his presence or absence in the events of any given origin determines whether the character associated with that origin dies or goes on to become the hero of the game's story.
 * The Dwarf Commoner is unable to escape the carta's prison and ultimately dies of starvation without Duncan's intervention.
 * The City Elf is either murdered by Bann Vaughn or dies in the ensuing riots in the alienage if Duncan is not there to lend him (or Soris, if the PC is female) weapons and once again invoke right of conscription to save the PC from arrest afterwards.
 * The Dalish Elf dies of darkspawn taint or becomes a Shriek if Duncan is not present to save the PC by making them a Warden.
 * The Dwarven Nobles, without the Wardens to chance upon, would have eventually died in the Deep Roads after being exiled.
 * The Mage PC is one of two characters with a chance of surviving the events of his or her origin without Duncan's assictance (by collaborating with First Enchanter Irving)--only to most likely fall prey to the event that troubles the Circle after Ostagar.
 * The Human Noble can survive without Duncan's assistance -- the youngest Cousland was already at the escape route, with no enemies nearby, when Duncan catched up, but one overheard conversation pegs all the Couslands as dead (on the other hand, so is your older brother if you are a Cousland, and he shows up perfectly fine at the epilogue). Regardless, without Duncan's presence the Human Noble wouldn't have been recruited as a Grey Warden and become the Hero of Ferelden.
 * The player-controlled Grey Warden serves as the nail in the DLC quest Darkspawn Chronicles, where, besides having the player control the darkspawn forces at the battle of Denerim, it shows to some extent what would have happened if the Grey Warden candidate from any of the origins never survived the joining ritual.
 * Nails eventually decide which of the three routes you take in the opening of Fate Stay Night. The best example is probably the choice leading to "Unlimited Blade Works": If Shirou recalls Saber before she slashes Archer, Archer is left unwounded and capable of participating in the Grail War at full strength. This in turn boosts Rin's position in the war, making her a competitor to Shirou instead of someone forced to rely on him as an ally (and also makes Ilya not notice Shirou). Thus, "Fate" and "Heaven's Feel" get locked out because everything begins snowballing from that decision . The difference between "Fate" and "Heaven's Feel", meanwhile, is decided by several nails during the opening that
 * The Playstation Portable version of Persona 3 hints at what would happen differently if the gloomy looking main character were replaced with an energetic looking girl. The answer?
 * Except Persona 3 Portable came out, so it's possible
 * Resident Evil 2 begins this way for the main characters:
 * Mobile Suit Gundam: Gundam vs Zeta Gundam has several What If storylines, but the one that best fits this trope is the Project Zeta story, where Garma Zabi surviving his fate from the original series, which leads to other characters killed by Amuro and the Gundam surviving, and eventually to Zeon winning the One Year War. But then it veers sharply into In Spite of a Nail, as the rest of the storyline is just First Gundam's events played out with older characters and Zeta Gundam-era mecha, even when several of those events already occurred even in the altered timeline.
 * There are a couple of other noteworthy alternate story paths; each of the three big heroes (Kamille, Amuro, and Char) has a storyline where they end up with the Titans, with varying degrees of believability . There's also a route that re-creates Chars Counterattack six years early and is a Kill'Em All Downer Ending.
 * In the Fire Emblem Jugdral timeline, the Big Bad has a Quirky Miniboss Squad named the Dark Warlords. Well, in the second game of said saga (Thracia 776), some of the Warlords are people from your own chara roster, who end up on the enemy side under different circumstances: getting killed in battle, not being recruited, or not being saved when they catch the Distress Ball. These charas are:
 * In Throne of Baal, the Solar reveals that Sarevok, the first Big Bad of the series, was also one of the Bhaalspawn children about to be sacrificed at the temple where Gorion found and rescued you. Gorion only had the opportunity to save one child, and he chose you instead of Sarevok.
 * In Mortal Kombat 3, Sub-Zero escapes from the Lin Kuei, and his friend Smoke is turned into a cybernetic ninja in his stead. However, Mortal Kombat 9 starts when the future Raiden (following the conclusion of Mortal Kombat Armageddon) sends a message to his past self warning him about the apocalypse that will occur. One of the changes that occurs as a result of Raiden's tampering with the past is that Sub-Zero becomes a cyborg instead of Smoke.
 * Additionally, Raiden's slaying of  in the reboot causes Shao Kahn to
 * Metaphorically speaking, the Big Bad of Radiant Historia spends much of his time pulling out nails--there's only one possible timeline that doesn't contain The End of the World as We Know It, and surprisingly small changes can prevent that timeline from coming to be. The main character, in turn, has the obligation to find the nails and hammer them back in.
 * in Cave Story. It's the single most important key to stop and get the happy ending. And it's incredibly easy to miss. If you don't get it, expect some rough times in the plot.

Web Comics

 * Erfworld Book 1 starts with this trope. The narrated opening scene shows the world's creators dropping one extra gem into a resource spot, which pays for an extra squad of units to go to field against Gobwin Knob's army, which is just enough to tip the balances in a heated battle, which means that Gobwin Knob's chief warlord gets an arrow between the eyes, which means that Gobwin Knob needs a new chief warlord before The Alliance shows up on their doorstep, which means that Wanda Firebaugh's bad day is about to get a lot worse. Judging from the way the story is going, this was just the start.
 * In a time-travel example: the Schlock Mercenary time-travel plotline in Book 6 invokes this trope, but isn't itself an example.
 * In Eight Bit Theater
 * Parodied here on Chainsawsuit.
 * Homestuck has innumerable offshoot timelines which all create elements necessary for the existence of the primary timeline. In one of those offshoot timelines, Karkat never runs the Mobius Double Reacharound virus.
 * In General Protection Fault, the Nega-Verse is what would happen if Nick and Ki were evil and acted on Nick's fantasy of taking over the world that was discussed in a brief arc early on. There are smaller examples, such as Nega-Trudy being as selfish as Trudy Prime until her mother's request for her to finde her sister (something Trudy Prime didn't hear about until after her Evil Plan failed) changed her for the better.
 * Parodied in this strip of Tom the Dancing Bug. Percival Dunwoody wonders if he caused the people of 2012 to turn into fly-men when he stepped on a twig in the Paleozoic; it turns out that he actually did it by
 * This Subnormality strip. If you look closely enough, the last panel nearly spells out this trope.

Web Original

 * Decades of Darkness has president Thomas Jefferson dying of a heart attack. His bumbling successor James Madison manages to anger the New England states enough so they leave the union and defend themselves successfully in a war, splitting the USA forever. The slave states get the majority and transform the rest into an expansionist slave-holding empire.
 * Look to the West`s very different world (Steampunk revolutionary France, a divided India, French Australia, loyalist America) all stems from George II tripping on the carpet at his coronation in 1727.
 * Well Meet Again's truly crapsack world solely emanates from the butterflies of a letter lost in the mail...
 * The 4chan imageboard /co/ had a thread that speculated what would happen if Bruce Wayne's parents never died. The result? Bruce and a certain fellow named "Jack Napier" end up becoming best friends, Gotham City goes through a golden age, and the readers cried manly tears.
 * 4chan's /tg/ board does this as well, some of the more recent ones, the crusades are won in the third crusade, the slew of changes involve a massive Kingdom of Jerusalem, The Library of Baghdad still stands, the reconquista never happened to its fullest extent, and the rising heresies move the reformation in a very different direction (Cathars, Flagellants and Waldensians rather than Anglicans, Calvinists, and Lutherans.) And these are the most normal things to come from it.
 * In the Chaos Timeline, Genghis Khan dying from a minor accident in 1200 prevents the unification of the Mongol tribes and their subsequent global conquests. The world becomes very different...
 * No Spanish Civil War in 1936 sees a (non-Stalinist) leftist Republican Spain not only survive the attempted coup in 1936 (with Franco critical to the thwarting of that plot), but become a junior partner of the Allies when World War 2 rolls along, having an anarchist for a president (Durruti), and great advances in Spanish influence in the arts of the mid-20th Century, all because Casares answered a letter from Franco that didn't go answered in real life.
 * The Union and Liberty timeline begins with a small change in Andrew Jackson's 1828 campaign platform and changing a few words in a John Calhoun speech.
 * 1983: Doomsday starts off with a certain Soviet Air Defense forces officer being assigned to a different bunker in September 1983. The officer who takes his place mistakes a false alarm for an American nuclear attack, leading to World War III.

Western Animation

 * Star Trek the Animated Series, "Yesteryear"
 * Futurama's "The Farnsworth Parabox," which features a universe "exactly like ours" except for the fact that the results of all coin tosses (or at least those in the recent past) are reversed. (According to the DVD commentary, it's a probability-maths injoke.) This leads to results like Bender being gold-plated, and Fry and Leela being married. Oh, also the colors are all mixed up, the sky is bright and swirly and the background music is a Theremin. How'd we forget that?
 * In the same episode, other parallel universes are visited, including a universe where everyone lack eyes, where their heads are bobble-heads, where everybody is a robot, a Roman Empire themed universe (complete with Roman numerals calculus) and a Hippie universe... However there do seem to be universes without the main characters, such as an octopus universe, a universe filled only with women and a completely frozen universe.
 * In "All The President's Heads", the gang travels back in time to The American Revolution to stop Prof. Farnsworth's ancestor David, who betrayed the Revolutionaries by counterfeiting money. After they stop David, Fry burns the counterfeit bills with a lantern he found in a church, which turned out to be one of the lamps used to signal the coming of the British (one if by land, two if by sea). Because of this, the British win the war and the gang come back to an America that's still under British rule.
 * Most of Timmy's time traveling adventures in The Fairly Odd Parents backfire because he doesn't realize until it's too late that he was the nail that made things go wrong; for instance, a Stable Time Loop involving Mr. Crocker's obsession with fairies.
 * Averted. Cosmo was the one who originally caused Crocker to become the way he was. Timmy just inadvertently caused it again except things also played out so Crocker now has better Fairy Hunting tech.
 * Not to mention the episode where he wishes he was never born. Tuns out his gender is a nail as well - his parents had a girl instead, and the life of everyone Timmy knows is better for it.
 * There was also the episode "Father Time" where Timmy, after melting his dad's trophy with Heat Vision, goes with Cosmo's crazy idea to go back and time to stop his father from winning the stupid trophy in the first place. When that happened, Timmy's dad gets the last place prize; an all expense paid scholarship to Dictator School, where he rules Dimmsdale in a dark future with creepy happiness and never got to marry Timmy's mom. So Timmy doesn't exist and lost his godparents (who vanish JUST before the revelation of Dictator School).
 * On the fifth season finale of Family Guy, Peter goes back in time to sow his wild oats and ends up not marrying Lois, who married Quagmire instead. Apparently, their not marrying Whew. Even after they "fix" the time-stream, the present still isn't the same: somehow, Roger the Alien ended up in their house.
 * Done in a Christmas episode of American Dad. Stan was sent back to 1970 to renew his Christmas spirit, and uses the opportunity to kill Jane Fonda, whom he blames for modern day liberalism. Though instead of killing her, Stan somehow convinces Martin Scorsese to quit drugs and thus Taxi Driver was never made; as a result, John Hinkley, Jr. never got obsessed with Jodie Foster, and never tried to assassinate Ronald Reagan to impress her. Reagan loses the election to Mondale, who immediately handed America over to the Soviets. The timeline is fixed by the end of the episode, except for Roger who became the inventor of disco, because he found a Best of Disco cassette that Stan accidentally dropped in 1970. And guns are easy to get because.
 * Subverted in the Fantastic Four 2007 animated series episode "Out of Time". While the Four are on a field trip to prehistoric Earth the Thing swats a mosquito, just as Mr Fantastic warns them not to change anything. The camera then pulls back to show he's already killed a dinosaur. Reed later assures him that this can't possibly have had the effect of Dr Doom ruling the world. But when the Torch gives cavemen the secret of fire, he's worshiped as a god in modern New York (although In Spite of a Nail, 15,000 years of Johnny Storm worship hasn't changed anything except the Statue of Liberty).
 * Everyone in New York chanting Johnny's name doesn't count?
 * Justice League, "A Better World" has an alternate universe where the League is a group of "well-intentioned" extremists, set of by the nail being The death of their Flash.
 * Beast Wars: G1 Megatron wanted to have any descendent with the possibility try this, with the nail being killing G1 Optimus Prime as the original transformers slept on prehistoric Earth, in order to sway The Great War into the Decepticons favor. Because of course, Optimus never dies, and when he does its always permanent. Beast Wars Megs very reluctantly tried this once, because he likely knew had he no other options, Unicron would've likely killed everything in this time continuity.
 * When the timeline is restored by the Maximals and the Heel Face Turn Blackarachnia, Megatron decides to try something a little different and goes after the primitive humans, as their help would later assure Autobot victory over the Decepticons.
 * Though there is still the question of if BW Megs absorbs G1 Megs Spark meaning in the "restored" timeline G1!Megatron is a sparkless shell.
 * He isn't. In a scene cut from the last episode, G1 Megatron is shown with his spark being restored to him. Presumably this was how he learned of the later technology that made the entire concept possible, resulting in a Stable Time Loop.
 * Freakazoid! inverts this. The titular hero ends up at Pearl Harbor in December 1941 and stops the Japanese from invading. When he comes back to the present, the world seems to be a better place: Sharon Stone can act, Rush Limbaugh's a "bleeding-heart liberal", EuroDisney is booming, cold fusion works, and there are no Chevy Chase movies. Just one catch:
 * Considering he likely had a hand in a fair amount of it, how is that a catch?
 * The Simpsons inverts it as well by having Homer's guardian angel Werner Klemperer show Homer what his life would have been like had he not married Marge. In the alternate timeline, Homer is rich and happily married to a beautiful woman, while Marge is the President, enjoying both success and popularity. Needless to say, this was not a successful trip for the guardian angel.
 * In a Treehouse of Horror sketch, the Butterfly Effect is taken to ridiculous extremes as Homer keeps traveling back in time and each time he steps on something, it changes the future in wildly differing ways. He comes to a future where the family is wealthy, his sisters in law are dead... but no one has heard of donuts. Homer runs back in time screaming (meanwhile it is raining donuts outside) and just indiscriminately squashes whatever is around him. He comes back to find everything normal... except everyone has forked, lizard-like tongues. He shrugs: "Close enough."
 * In the Phineas and Ferb epsiode "Phineas and Ferb's Quantum Boogaloo", future Candace travels back in time and helps her younger self bust Phineas and Ferb on the first day of summer. When she returns to her own time, she discovers this act has lead to the tri-state area becoming a dystopia ruled by Doofenshmirtz. She then has to travel back in time again and stop herself from helping her younger self from busting her brothers.
 * Specifically, Perry & Doofenshmirtz's fight failed to eliminate the evidence of the rollercoaster, allowing Candace to show it to her mom. Also, Perry was injured in said scuffle, which put him out of action long enough for Doof to take over.
 * An episode of Captain Planet and the Planeteers ("Two Futures") shows how the world would have been in a HUGE mess if Wheeler hadn't accepted his ring.
 * Which brings the question: Why didn't they get somebody else to accept the ring?
 * Because then it wouldn't be Wheeler's fault.
 * Gaia says in the intro that they are "five special young people". Apparently they're very special indeed.
 * A sketch in Robot Chicken that shows what would happen if jerks got time machine. It ends with one jerk stopping Nazism from ever getting started by humiliating Hitler, thus becoming a Hero with a Time Machine
 * A parody of Popeye where an angel shows Whimpy what would have happen if he did not exist when he attempts suicide. Popeye and Brutus are friends, Olive has huge breasts, Goon gains a PHD, there is no war and all hamburgers are free. The angel, seeing that he failed, pushes Whimpy off the bridge.
 * In My Little Pony Friendship Is Magic, as realized in the episode "The Cutie Mark Chronicles", Rainbow Dash's first Sonic Rainboom actually indirectly caused all the rest of the Mane Six to get their own cutie marks.
 * In the Rated A for Awesome episode "Thera's Date With Destiny" Noam's new future-predicting invention shows that Thera will be going to prom with Chet. They try to change the outcome, but each attempt either has them start dating even sooner or she goes to prom with someone else she doesn't want to go with. That is until the last vision, which doesn't show her date's face, but it's hinted to be Noam. Each vision after the first also shows her with a different hairstyle.

Real Life
"Heydrich's last words: "The world is just a barrel-organ which the Lord God turns Himself / We all have to dance to the tune which is already on the drum.""
 * Despite having multiple redundant systems and some of the most stringent inspection and safety standards in the world, almost all airline crashes can be distilled down to this trope.
 * United Flight 232 in 1989 crashed because of a microscopic, metal-fatigue induced, hair-line crack in the compressor fan of its #2 engine. This crack caused the compressor fan to fail catastrophically, causing shrapnel to sever ALL THREE hydraulic system lines, causing complete loss of flight controls, causing the plane to crash and 111 passengers to lose their lives.
 * All three hydraulic lines ran through the same narrow area, though. Three lines, but a single point of failure.
 * AeroPeru Flight 603 took off with a small piece of tape covering an air pressure sensor (it had been put there to protect the sensor while the plane was cleaned). This prevented the sensor's operation, leading to the altitude and airspeed data to become confusing. This prompted the flight computer to begin issuing contradictory warning messages. This led to the crew becoming confused and disoriented. The end result was a CFIT (the crew piloted the aircraft into the ocean, thinking they were at several thousand feet altitude when actually they were skimming the waves). Over a hundred people died due to a piece of tape.
 * The proper way to look at it is that not despite, but because of stringent inspection and safety standards all airline crashes come down to this trope. Single isolated failures and mistakes have mostly been eliminated as cause of accidents.
 * The Deepwater Horizon oil spill of 2010 could have easily been averted if not for a single dead battery in the emergency shut-off valve.
 * The Titanic was probably one of the very few disasters that may have been prevented if someone did nothing. Although plenty has been said about lifeboats, speed, and paying attention to reports, what actually doomed the Titanic was the iceberg scraping across the side of the ship, rendering the water-tight compartments useless. If the FO had merely ordered reversed/stopped engines, or even just maintained speed without changing course, the Titanic would've remained afloat.
 * There was a closer ship, but the radio operator had gone to bed and didn't hear the distress signal. And the crew thought the flares were just fireworks... Wow, there were a lot of nails that could have salvaged this one.
 * Said ship was the SS Californian.
 * And, the criticism of the press of the world notwithstanding, it was too small and too slow to have been of much good anyway. Its few lifeboats could have been used to pick up survivors struggling in the water...but then again, many of Titanic's boats were half-full and still did not pick up survivors.
 * The Norman Conquest of England had the weather as one of its nails. William of Normandy's invasion fleet was assembled and ready to launch by August of 1066, but was delayed by unfavorable winds until late September. Between August and September the Saxon king Harold Godwinson faced two crises that depleted his forces: the term of his conscripted troops (who were mostly farmers and needed to tend to their harvests) came to an end, and Harald Hardrada of Norway invaded England from the North. By the time William's fleet reached England, Harold Godwinson's army had been reduced to a fraction of its former strength. Had the weather cooperated and William had been able to launch his fleet earlier, he would have faced Harold at full strength. Even with Harold's forces depleted, William faced stiff opposition and his troops were often outnumbered on the battlefield. Facing a full-strength Saxon army, William would likely have been defeated, and the Norman Conquest would never have taken place.
 * Although it was just the catalyst, the murder of Franz Ferdinand (Archduke of Austria-Hungary) by a Bosnian Serb nationalist led to World War One. This devastated Germany so much that it gave Adolf Hitler the opportunity to gain popular support, which led to World War Two. It gets more interesting, too. For instance, the assassin got his opportunity because Archduke Ferdinand's driver took a wrong turn, and struck while he was pausing to get his bearings. Also, the Archduke's life could probably have been saved with prompt medical attention to his wounds, but this was prevented by his heavy and ornate uniform which he had had himself stitched into so that it would stay as neat as possible during his public appearance.
 * It's highly doubtful that the Archduke's life could have been saved even if his staffers and doctors had been able to get his tunic off immediately. The bullet had crashed through his neck, severing his jugular vein, damaging his trachea, and lodging in the spine. In point of fact, it wasn't removed during the autopsy because doing so could have damaged blood vessels that needed to be kept intact for the process of embalming the body (which suggests that even if surgery could have been performed, it wouldn't have been any use to remove the bullet). Duchess Sophie, the Archduke's wife, couldn't have been saved in any case; her fatal bullet severed the main artery and vein in her lower abdomen, causing death within several minutes from massive internal bleeding.
 * It's almost as if there were a bunch of time travelers trying to interfere with this event and prevent WWII...
 * And the assassination would not have taken place if not for a chain of events. Previous attempts had injured members of the Archduke's party. He was on his way to visit them in the hospital when the assassin just happened to be having a sandwich as the motorcade drove by and had to stop to get their bearings.
 * The whole assassination was a comedy of errors. Of the group involved in the plot, most of them didn't even do a single thing as the motorcade drove past them. Cabrinovic, the young man who threw the bomb/grenade at the Archduke's car, was the only one to actually take action before Princip.
 * One of the Archduke's staffers had taken up a position on the running board of the automobile on the Archduke's side precisely to provide a human shield if anyone else tried to kill Franz Ferdinand. It was a perfectly good idea, except that Gavrilo Princip attacked from the other side of the vehicle.
 * Adding insult to injury, the Archduke, at least as it seems, wore a primitive form of bulletproof vest (multiple layers of tightly-woven silk on a metal wire frame) under his uniform, perfectly able to withstand the weak .380 ACP bullet. To no avail, as the bullet struck him in the neck, above the vest, flying downwards.
 * Duchess Sophie's (Franz Ferdinand's wife) death was a completely avoidable tragedy. The Archduke actually gave instructions for her to be taken back to the hotel where they were staying while he drove on to see the people who had been wounded in the bomb attack, but she refused to leave her husband's side as long as he chose to risk himself in public. Gavrilo Princip himself testified at his trial that he had never intended to shoot the Duchess; in fact, he wasn't even looking to see where he was aiming when he fired the fatal shot (which was intended either for the Archduke or for General Potoriek, the military governor of Bosnia).
 * Counter to this, the "Mad Monk" Rasputin wielded great influence with the Tsar at the time, and had both the desire (as explained in his writings) and the means to persuade him not to join in the growing conflict (on the side of Serbia, which was facing reprisals from Austro-Hungary). But Rasputin was not able to hurry to the Tsar's side because he was recovering from being stabbed by another monk, a rival for influence in the tiny monastic sect that they belonged to. If Russia hadn't entered WWI due to Rasputin's intervention... at the very least, Russia wouldn't have destroyed its economy trying to fight the war, the Tsar wouldn't have abdicated, and there would have been no Bolshevik Revolution. Added to that, however, Imperial Germany wouldn't have been able to declare war on Russia in support of Austria-Hungary, leaving Great Britain with little reason to get involved either. With major players sitting it out, the whole thing could have finished up as Austria-Hungary stomping on Serbia for a bit, with the rest of Europe issuing firmly-worded declarations.
 * On the other hand though, this might just be an in-spite-of-a-nail situation; we can't say for sure one way or the other of course, since we can only see one reality at a time. The entangling alliances throughout Europe existed long before Ferdinand's driver took a wrong turn. So did the anti-monarchist sentiment in Russia. Given the conditions as of 1900, at least one devastating war in Europe and/or a communist revolution seemed almost inevitable.
 * Inevitable is a good way to put it, but it could have easily been very different.
 * The nations who participated had been expecting something to start a major war years before the assassination.
 * Several crises in the decade or so previous to June 28, 1914 came very close to touching off a major European war: e.g., the Dogger Bank Incident, Fashoda, the Moroccan crisis of 1911, the original Austrian annexation of Bosnia-Herzegovina in 1908. In those years, Europe was a powderkeg just waiting for the proper spark.
 * Without Germany declaring war on Russia, the chances of a Bolshevik revolution would be slim: Lenin was stuck in Switzerland (IIRC) prior to the war and got carried to Russia (in a sealed train lest he escape into Germany) as a bid to force the Romanovs to withdraw from the war. And without the Bolsheviks, the Jews and Communists never would've been mixed in popular thought, meaning the latter's talk of exterminating millions of people couldn't've been used to fuel the antisemitism the Nazis used to rise to power -- which means that millions of lives might've been saved if Lenin hadn't crossed somebody's mind. On another note, had Wilhelm II not been talked out of unrestricted submarine warfare early in the war for fear America would enter (despite the fact that the US was incapable of doing anything meaningful for some time), Britain very well could've been forced to surrender.
 * Naturally, it's hard to say for certain, but the Second World War saw four of five German casualties fighting the Soviet Union--and it wasn't the fist German invasion of Eurasia since 1900, so it could have happened regardless. The Soviet Union, with its technocratic (by the standards of the time) leadership and complete absorption in rapid industrialization, had an infinitely better infrastructural and industrial base to fight a western invasion than the 20th century Romanov Monarchy and its attempts at industrialization. And with two, three, or four times as many German soldiers on the western front...
 * During the battle of Marcoing, a wounded German soldier walked into Pvt. Henry Tandey's line of fire, but he couldn't bring himself to shoot the man, so he let him go. That soldier was later found out to be Adolf Hitler.
 * Similarly the American Revolution would have mostly likely failed if not for the honor of a British soldier who had General George Washington in his sights and did not fire because it would have meant shooting the general in the back.
 * And without Benedict Arnold's efforts in winning the Battle of Saratoga, many historians believe the war would've taken a turn for the worse.
 * World War II was full of events that could have changed history if their outcomes had been different, but arguably the biggest one happened before the war even started: The Battle of Khalkhin Gol in Mongolia, the decisive battle in the Soviet-Japanese border wars, where Zhukov won using the blitzkrieg tactics before the Germans did (shame that the Soviets wouldn't use it for a while). Believing that the strength of Zhukov's forces was a standard average of the whole Red Army (while in reality Zhukov brought the best of the best with him) the Japanese High Command abandoned the North Strike Group Plan on Russia and instead concentrated on the South Strike Group plan attack on southeastern Asia, which led to Pearl Harbor. And during the German invasion of Russia, the Japanese said they would help only if the Germans would cut off the Russian reinforcements by taking over cities on the Volga river, which included Stalingrad. Knowing that the Japanese wouldn't attack, the Russians transfered their forces from Siberia to fight with Germans, and the rest is history.
 * A smaller event occurred during D-Day after the Ox & Bucks glider troops had captured what became known as "Pegasus Bridge". A sergeant managed to destroy a German tank that was heading for the bridge with a PIAT shot, blocking a key junction and forcing the other tank to retreat, as well as creating a big fireworks show via the ammo cooking off that gave lost paratroopers their bearings and convincing everyone there was a major battle taking place, causing the Germans to believe the forces were much larger there than they were. In his book on the matter, Stephen Ambrose argues that had he missed, it might have led to the loss of the 6th Airborne Division or even the failure of Operation Overlord.
 * Operation Barbarossa was a whole string of these. If Hitler hadn't diverted AGCentre's panzers south, he could probably have taken Moscow; if he'd kept them down there, he could well have taken the oil fields, which might also have been achieved if he hadn't tried to take Stalingrad at the same time (or alternatively he could have pushed on Stalingrad). If he hadn't been so pig-headed, he might have ordered a break-out of the Stalingrad divisions before the Russians managed to properly encircle them.
 * It's unclear whether this is the trope, or Russia was simply too big a piece for Hitler to chew. Besides, if Hitler didn't divert the AGC south, he would have been advancing to Moscow with a million Soviet soldiers in his rear. The forces in the north were giving him enough trouble as it was.
 * As per the above Harry Turtledove entry, the entire outcome of the American Civil War could have gone differently if some random Union scout hadn't found a lost copy of Robert E Lee's Special Order 191: the Maryland invasion plan. The North had the advantages in manufacturing and troop size, so the only real chance the South had to win the war was to bring it to a swift and decisive conclusion, which may well have happened if Confederate troops had captured Maryland and got within striking distance of Washington. With the interception of the battle plans, the Union now had detailed knowledge of Confederate military strength and planned maneuvers, and was able to halt the invasion at Antietam. Although the war dragged on for another few years afterwards, the Confederacy never regained the momentum they had at the outset and eventually lost by sheer attrition.
 * For three straight years the Los Angeles Lakers dominated the NBA, winning its championship from 2000 to 2002. Shaquille O'Neal, its leader and driving force, injured his toe during the season. His injury led to the Lakers losing several games and the home court advantage during the playoffs, each playoff series decided by a best of seven margin. As a result the San Antonio Spurs got the home court advantage and by the time the Lakers started getting back to their winning ways, it was too late, and they lost the series 4-2. All this due to an injured toe.
 * For want of a toenail?
 * The Greco-Turkish War might have been very different if the previous Greek head of state had not died from an infected bite from a pet monkey. This war was one of the major conflicts leading to the formation of the Republic of Turkey.
 * The Kennedy assassination and the murder of the assassin Lee Harvey Oswald himself both resulted from an unlikely chain of lost nails.
 * The assasination of SS-Obergruppenführer Reinhard Heydrich in Prague could happen only by an unexplained stroke of bad luck, as he had already seen the Czech assasins trying to shoot at him with a crappy Sten gun which jammed and shouted to his driver-bodyguard to speed up. As the driver did not understand at first the order and slowed the car down, the assasins could drop the bomb they carried as a secondary weapon, fight back with submachine gun and pistol and escape unscathed (not for long). Had the plot failed, Heydrich would have survived for sure, and his brilliance in running the German secret services might have changed the outcome of the war.


 * In one of Lewis Lovhaug's videos, he explains because of paranoia over the Colombine school shootings, he was suspended from school over something he said. His parents blamed the school and allowed him to treat his suspension as a vacation rather than a punishment, so he got an opportunity to watch Sailor Moon (figuring that if he was going to heckle his brother over it, he should know if it was worth mocking). This led him to becoming a fan of anime and eventually creating Atop the Fourth Wall - without the suspension "Linkara" wouldn't exist!
 * On September 26, 1983, a Soviet early warning station detected 5 inbound ICBMs. Colonel Stanislav Petrov, the man in charge of the station, decided it was a false alarm and did not report it to his superiors. Had someone else been in charge, and had they not realized it was a false alarm, well...
 * Perhaps a little comfort can be found in the fact that anyone with even general knowledge of nuclear war (such as a person in charge of a station dedicated to detecting missile launches) would understand that no one would launch just five ICBM's as a first strike...they'd launch EVERYTHING.
 * Similarly, in 1979, at the American NORAD aerospace warning control center, a technician stuck a training tape depicting a full nuclear exchange with the Soviets into the main computer without remembering to hit the training button. As a result, NORAD interpreted World War III from within its own computers. Within a few minutes, American aircraft were loaded with nuclear weapons and launched, and missiles were prepared for a full retaliatory strike. For want a switch, the world might have been lost?
 * Also a Professional Wrestling example, but since it was a major part of real life, it belongs better here. At a live event at Madison Square Garden in May 1996, The Kliq (Triple H, Shawn Michaels, Kevin Nash, and Scott Hall) all hugged each other in front of the audience in what became known as the "Curtain Call". While it was not a problem for Michaels and Hall to hug (as both were faces), when Triple H and Nash joined in the hug, this scandalized the WWF (because both were heels), and this was seen as breaking the suspension of disbelief. While Michaels was not reprimanded because he was the WWF Champion, and Nash and Hall were leaving to go to rival WCW, this left Triple H as the sole member to take the blame. He was demoted from being a championship contender to the mid-card, and while originally booked to be in the finals at that year's King of the Ring, the MSG incident prevented that. Who took his place, and the push that went with it? Why, none other than Stone Cold Steve Austin, then a former WCW midcarder and current WWF midcarder, who went on to win the KOTR, jumpstart the Attitude Era, and help the WWF win over WCW in the Monday Night Wars. If the MSG incident had never taken place, then who knows what the state of the professional wrestling business would be today?
 * The butterfly effect works like this, except vastly more extreme, and with worse story-telling. If you move one atom a Planck length to the side, it will move the atom it hits slightly more, growing exponentially until the weather patterns change completely. In addition, brownian motion (the motion of atoms bouncing off of each other) will change very quickly, which will change which sperm gets to the egg in every conception, meaning that every person who is born from then on will be different.
 * Except of course it doesn't work like that, because random thermal motion at the molecular level will tend to damp the subatomic variations out, NOT amplify them. The system has a certain amount of negative feedback built in. Worse story-telling indeed!! (E. E. "Doc" Smith's Lensmen series actually lampshades this - the Arisians have long been aware that random, statistical variations occur on the microscopic level and predictable mechanistic ones on the macro, and their ability to predict the future (largely successful in-universe) depends on their competence to know where the interface of these is and what's happening there. Even though they're almost Godlike, they admit they are not competent to solve the problem fully.)
 * Source? Wouldn't differences getting damped out violate the law of increasing entropy?
 * Also, while a butterfly can easily change the weather, it can do little against the climate. A butterfly won't cause a hurricane in an area that doesn't get hurricanes.
 * An urban legend tells of a young man out to make his fortune in the city. He applies for a job in a brothel as a bookkeeper, but is turned away when he reveals that he can't read or write. The madam takes pity on him though, and gives him a couple of apples for his lunch. But instead of eating then, he sells them. He then manages to get a good deal on some more apples, sell those at a profit, and repeats the process...until eventually he owns his own fruit stand, then his own grocery store, then finally his own produce company. In a few years he's the biggest produce magnate in the country and is voted Businessman of the Year. When interviewed on the national news, the interviewer is shocked to discover that the man is still illiterate. The interviewer asks him if he did all that without being able to read or write, what could he have accomplished if he was literate? The man considers the question for a moment, then cheerfully responds "Well, I guess I would've been a bookkeeper in a whorehouse!"