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The 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 Buffyverse 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 he died, 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....
In the 100th episode, 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.
They play with it again in another episode in which something so minor as Piper randomly picking between turning left or right at a critical moment resulted in some pretty major messing with fate due to the circumstances. By the end of the episode this had snowballed into her sisters getting killed by a demon from the future. She fixes it simply by going back to the past and telling herself to go the other way when it comes up.
Community: The entire 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 an alcoholic, Troy losing his larynx, and Britta dyeing a strip of hair blue) until the real timeline where Abed catches the six-sided die before it rolls and instead makes Jeff go get the pizza himself.
Dad's Army: Features Captain Mainwaring (attempting to be philosophical) reciting part of the relevant poem, only for Lance-Corporal Jones to interject at the end:
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 —
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?
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, the Doctor dies, lots of other companions and allies die, London explodes, and the holocaust happens in England. Also, all of reality is unmade. But this being Doctor Who, that could have happened, anyway.
It's also discussed, with the Doctor explaining, once things are normal, that for most people tiny changes like this really do only result in tiny changes to the overall timeline, if any at all. Donna's a special case.
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 to unmake reality.
In this case, however, it's less "if Pete Tyler survived then flying dragon demon things would try to unmake reality", and more "Rose changing her own past has already screwed up reality, and the flying dragon demon things are trying to keep the rest of the universe from finding out".
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".
As of the season 4 premiere, 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.
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.
"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. After a week, Frasier's lives meet at the same point, showing no matter which choice he made, he ended up at the same destination.
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.
Martin: You know, I don't like this red bowl, it's throwing everything off!
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 "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.
In season 5, Peter manages to do this using the observers' technology, like blocking traffic for a minute in order to prevent two people from meeting.
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.
Heroes: The 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.
How I Met Your Mother: In the episode "Lucky Penny," Ted and Robin miss a flight, then recount the incidents that lead to the incidents that lead to missing the flight, which (Future-Ted points out) itself has a significant effect on his life. The nail was the titular lucky penny.
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 Yuto Sakurai, AKA Zeronos as a teenager, resulting in a reality that seems to be happier overall. Airi, no longer burdened by her missing fiancée, is happier and more vivacious; her coffee shop, now frequented mostly by women, is much more successful; her former suitors have moved on and are actually in successful relationships; and Deneb is happily contracted to Ryotaro. The series' method of handling Time Travel soon restores the dead character to life, and Kai's attempt to sway Ryotaro fails.
Lets Go! Kamen Rider: This is a major plot point of the upcoming 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 season 6, 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 was purgatory all along!
Or it worked but, destroying the source of all life created a purgatory-like realm.
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.
Episode "If Boys Were Girls (during season 4 - the pregnacy season) has Lois at the mall with the boys. When they start their usual troubles, Lois phases into a fantasy world where all of their children are girls. It starts out nice and pleasant but later it turns out that they're none the better. Even worse: As they are female, themselves, they know much better how to deceive their mother. Lois still wishes for the new baby to become a girl. At the end of the season, Jamie is born. Some episodes later, it is revealed that Jamie is male.
Married... with Children: Subverted Trope in a Christmas Episode. In a parody of Its A Wonderful Life, an angel (played by Sam Kinison) shows Al what the lives of his family would be like if he had never been born. However, it doesn't turn out as he'd hoped: Peggy is a wealthy socialite that is Happily Married to a handsome guy and is a wonderful mother, Kelly is is a college student genius and child prodigy in French and writing, while Bud is also a genius and a ladies man on top of it. To make things worse, Steve and Marcy are downright miserable in this reality, being much like Al and Peg in the real world. Still, this isn't a failure for the angel; being the misanthrope he is, Al decides that he would rather be miserable himself than see his family, the source of his misery, happy.
Primeval: In the Season One finale, 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.
One episode 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? A cigar in Al's glovebox, belonging to his best friend at the time, who turned out to be the real murderer.
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. And everybody thought he was supposed to prevent the 1960 U-2 Incident.
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 the question he asks her because the wind blew some pages while they weren't looking. Interestingly, Al seems to notice while he was in court; and the shift occurs right when the decision was being read.
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. Noteworthy in that Ace was the one held back - the humiliation drove him to fight back and improve himself.
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.
Revolution: "The Night The Lights Went Out In Georgia" reveals that the Georgia Federation not only still has a trading fleet of tall ships, but is in regular communication (and possibly a military alliance) with England. If only Maggie had travelled there instead of the Monroe Republic as well as survived past "The Plague Dogs", she might have made it home to her kids after all....
The 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, except for one patient, who dies at the end of both.
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. The Janitor saw JD drop the penny, and was hoping he would fess up so they could be friends. JD didn't confess because he didn't want Janitor mad at him, so they could be friends.
Seconds Before Disaster: This documentary series is centered on this trope and lampshades this a lot:
Narrator: "A chain of critical events that led to the X disaster..."
This was the most common plot in the first few seasons, 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.
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.
The 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 Stargate Continuum had Ba'al causing this intentionally, with plenty of resulting changes. 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. The captain was Mitchell's Identical Grandfather, and he hadn't had kids yet. 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...
Several alternate SGCs are seen to have been defeated by Apophis' invasion at the end of season 1. In the case of "There But For the Grace of God", the SGA is crippled by having had no Daniel Jackson on the expedition to Abydos, and therefore never learning the Goa'uld language.
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.
"Yesterday's Enterprise": the Enterprise-C was originally destroyed while defending a Klingon base from a Romulan attack. In this episode, a temporal anomaly throws it forward in time to the time of the Enterprise-D. This changes the timeline into a Bad Present where the Federation is at war with the Klingons and on the verge of losing. The Enteprise-D is now a warship, Worf is absent for obvious reasons and Tasha Yar is still alive. Guinan is aware of the changes and convinces everyone to defend the Enterprise-C while it goes back to the past, knowing that everyone involved will be killed (and with Tasha taking the place of the C's tactical officer).
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, one has the Bajorans becoming a militaristic empire after defeating Cardassia, 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.
Star Trek: The Original Series: "City on the Edge of Forever". Specifically, the event here is a woman in the 40s not getting hit by a car and proceeding to lead a massive peace movement that keeps the US out of World War II for several years. This giver the Nazis time to develop nuclear weapons and win the war.
The "Year of Hell" two-parter involves 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 due to this comet being involved in seeding most of the inhabited planets in the sector billions of years ago. The captain might seem like a bit of a hypocrite for pointing out Chakotay's mistake, but the whole thing is about him trying to fix the mistake he made in the first place. 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.
In the Season 2 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 every person he, Sam, and/or their father had saved while hunting had died. And then it turns out to be All Just a Dream caused by a djinn.
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.
Season 5's finale "Swan Song" opens with a narration from Chuck about the significance of Dean's 1967 Chevy Impala. He even declares it to be the most important thing in existence. Earlier in the season, Dean had been shown a vision of the future in which they were unable to prevent the apocalypse. The experience convinced him to stay closer to Sam, which in turn caused him to be present at the showdown between Michael and Lucifer. Dean didn't actually do anything to make any difference; simply his and the Impala's presence was enough to give Sam the strength to overcome Lucifer long enough to lock him away, thereby preventing the apocalypse.
A Warehouse 13Christmas Episode has Pete play with an Artifact (as usual), resulting in him finding himself in a world where he was never born. As a result, Artie never made his escape during the Blood Stone incident in the pilot and was caught by Myka on the charges of attempted assassination. Myka was never made a Warehouse agent and had no reason to go back to visit her parents; thus, her father died of a heart attack having never mended fences with her. With Artie out of the way, McPherson manages to convince Mrs. Frederick and the Regents that he was framed by Artie and is reinstated. He continues acting in his own interests but, being the top agent, manages to keep it hidden from everyone. Oh, and Claudia is still in the mental hospital, while her brother is trapped between worlds.