A Sub-Trope of For Want Of A Nail, when the changes between two versions of history seem unlikely (even by chaos theory), or even outright impossible, from the particular change in the past. Even if the timeline is set right, there might be some changes that just don't make sense, even by how things were set right.
The key is that you cannot see any direct explanation for the changes. If you cut down a tree in the past and come back to the present to see the stump just sitting there (or maybe replaced with a new, smaller tree), that is not this trope. That's just continuity. This trope is if you knocked a tree down in the past, and upon returning to the present day, the world has devolved into The Apunkalypse, your parents are now otters, and some nearby bench was repainted. Just knocking down a random tree shouldn't have resulted in any of that... except maybe the repainted bench. Maybe it was done in honor of the tree.
This used to be played straight, but as you can tell from our example, you're now far more likely to see it Played for Laughs. By the Turn of the Millennium, most serious time travel stories began making an effort to have any timeline changes be the result of logical, explainable cause-and-effect to avoid making the Timey-Wimey Ball even worse than it is. That isn't to say this trope isn't still done seriously on occasion, but unless the rest of the story is good enough to keep that much needed Suspension of Disbelief in place, you can fully expect audiences to raise an eyebrow if a writer attempts to do so.
Compare Butterfly of Doom, where an innocuous action done in the past results in major negative changes to the present which may or may not be absurdly unlikely. Contrast In Spite of a Nail, where certain things remain constant no matter what changes are made. Alternate History Wank may be particularly vulnerable to this, as the author often starts out with an implausible world they want to create and work backward from there.
- Flashpoint (DC Comics) has this problem as various individuals are dead and things have led to a pretty big Bad Future. Possibly justified since time was not "changed", but rather broken, meaning things were not meant to be this way. The resolution of the storyline, which reboots the universe as the New 52, introduces still more examples of this problem. However, Rebirth reveals that reason why and it's a doozy. Dr. Manhatten of Watchmen fame interfered with the DC Universe and among other things, removed from existence the Justice Society of America, starting with Alan Scott.
- In one issue of PS238, Tyler is shown glimpses of alternate timelines in which he was born with different superpowers. In most, the visible changes make sense as consequences of that Tyler's powers, but there's one where everybody is in the middle of a crisis that starts up ten issues later in the main storyline, and there's no obvious reason why it should happen sooner in the other timeline just because Tyler has gravity-manipulating powers.
- In Star Trek (IDW), the arrival of Nero in 2233, changing everything from that point, somehow results in the events surrounding the USS Archon's crash on Beta III being drastically altered. This happened in 2167.
- Many issues of Marvel's What If? fall into this trap, where For Want Of A Nail takes place... along with completely unrelated events that didn't take place in the original story. Essentially some stories become "What if... (event happened differently) but these things were also different?"
- In "What If Wolverine Led Alpha Flight?" where the X-Men depart and leave Wolverine with Alpha Flight, but then are randomly shot down by an intercepting aircraft and killed.
- In "What If Captain America had formed the Avengers?", which despite being the direct sequel to a much more coherent issue, asks you to make a lot of leaps in logic to make sense of it all.
- In "What If the Hulk killed Wolverine" where the death of Wolverine creates a cosmic imbalance favoring chaos and thus the super villain the Adversary is free to imprison Roma and go on to kill many other X-Men, something he never did even when he fought them in the mainstream continuity.
- In "What If Professor X became the Juggernaut" the Fantastic Four decide to randomly attack Xavier and the X-Men after the latter trashed the Sentinels that first attacked them, handwaving that Reed Richards was friends with Bolvier Trask, the maker of the Sentinels, resulting in them all losing their powers due to a device that Xavier makes.
- In "What If the Marvel Super Heroes had Lost Atlantis Attacks", Set contaminates the world's water supply turning nearly everyone into serpent people (including most of the remaining super heroes and villains), except for about eight random superheroes and villains. No reason is given why these particular eight never drank the water, and why others did, other than for the purposes of the story. Drinking water even transformed Warlock of the New Mutants into a serpent man, even though he's a techno-organic being that doesn't know what liquid water is! You'd think Warlock would be one of those immune, if anything!
- Back to the Future Part II includes an alternate-1985, in which Richard Nixon is running for a fifth term and The Vietnam War is still ongoing. All because Biff Tannen learned about the outcomes of all sporting events and made a fortune from gambling! The comic Biff to the Future makes a valiant attempt at explaining this, starting with Biff buying the Washington Post in order to control the Hill Valley Telegraph, and immediately sacking Woodward and Bernstein.
- It's a Wonderful Life mostly avoids this, since most of the differences in the Bad Future where George was never born are things George, as one of the community's most influential voices, would reasonably have been involved in. The one real exception is that it's snowing in the original timeline, but not in the Bad Future. It was most likely done as a visual shorthand for the audience's benefit, but it does make you wonder exactly how George's removal from history could have changed the weather.
- Dirk Gently's Holistic Detective Agency has a perfectly lampshaded use of the trope. Early in the book, it turns out that in bringing the coelacanth from prehistory to the early 20th century to be rediscovered, the dodo went extinct, and that was the end of a short chain of similar changes. At the end of the book, after the main characters go back to the dawn of life on earth, Dirk finds out that his secretary was still working for him, and a cat that he'd spent the last eleven years searching for had never gotten lost in the first place.
- In The Hitchhiker's Guide to the Galaxy, using the Infinite Improbability Drive on the starship Heart of Gold tends to cause this throughout the universe. That being one reason why it is replaced with the Bistromathic drive.
- In Robert Silverberg's short story "Needle in a Timestack", time travel is common for holidays, so minor changes (your car was a grey Toyota, now it's a silver BMW) are just "the little annoyances of modern life". Unless your wife's first husband is trying to undo his divorce.
- The Power Of Un uses this as well — the carnival is subtly different (paint colors and such) even though the timeline started changing just that morning.
- In The Science of Discworld II, the mages' meddling with time (trying to make sure William Shakespeare is born and becomes the great poet he is supposed to be) accidentally results in the first potato being brought to Europe.
- Snapshot: Even the smallest action by the Snapshot detectives can touch off a chain of events that will completely change what happens in the Snapshot. Davis mentions a time he stayed in his room all day to avoid causing Deviations; he slammed a door too loudly in the morning, waking up a woman who was supposed to sleep through a job interview. The results are so unpredictable that the Snapshot detectives are never blamed for triggering Deviations.
- Season 4 of Eureka: Despite removing one of the town's founders from the timeline in 1947, the biggest changes that occur in the resulting future are that a few people have different jobs, Henry's married, and Jo's not dating Zane anymore. And Allison's kid is no longer autistic. And a statue has changed materials. Strangely, nothing has changed for Beverly, even though her father was jailed because Grant wasn't there to back him up. Near the end of the series, another change is shown: apparently, Henry is a Consortium agent in this timeline.
- Many small changes to the barracks and a few minor discrepancies in continuity in Lost have been theorized to be because the time-travelling survivors altered some minute details of the past when the island was shifting through time. These include Rousseau forgetting Jin and Aaron's birth being slightly different.
- In Primeval, leaving a couple of Future Predators in the past somehow changes Claudia Brown into Jenny Lewis, gives the team a new HQ, and alters Connor's dress sense. It also added a minor Big Bad named Leek. Interestingly, killing an entire tribe of proto-hominids in the distant past did absolutely nothing.
- Red Dwarf uses this in "Timeslides", when the last change to the timeline puts everything back how it was except that Rimmer is alive. He dies seconds later and the change in his backstory is apparently forgotten.
- Saturday Night Live's "Timecrowave" sketch, with host Alec Baldwin, has this crossing over with Butterfly of Doom ad absurdum. The titular machine uses time-travel tech instead of microwaves to heat up TV dinners, sending them back to one minute before they were put in. But Alec receives a chicken dinner a minute before he put in a roast beef one, and all hell breaks loose in waves. Among the changes: Alec switches from white to black and back again, Kristen Wiig grows a mustache, a giant tabby cat appears outside the kitchen window, and (you guessed it) we see that every house on the block has a swastika-adorned flag hanging in front.
- Stargate SG-1: After SG-1 is sent back in time they start a revolution against the Goa'uld, the problem is that the Goa'uld take away the stargate and the alternative SG-1 has to go back in time and put things as they were. When the timeline is corrected, the only difference is that Jack's lake now has fish.
- Winning the game in Dark Fall: The Journal undoes something unnatural that'd happened at the Station Hotel in the 1940s. In the sequel, Dark Fall: Lost Souls, the same hotel has suffered severe bomb damage from WWII, which hadn't been evident in the previous game. Unless writer Jonathan Boakes is implying that the unbound dark entity had somehow deflected a bomb in the first game's timeline, and did so several years before this Unseen Evil actually broke loose...
- Life Is Strange goes pretty wild with the consequences of time travel. Max saving her schoolmate from getting shot results in the entire town being destroyed in a freak tornado a few days later. At least the game is aware that there's absolutely zero logical way to get to that result, and characters theorize (it's never fully confirmed) that it was caused by the timeline itself getting freaked out by the change.
- Mega Man Battle Network takes place in an Alternate Universe from the Mega Man (Classic) series, where more research was poured into networking rather than robotics, and as such most of the NetNavis in Battle Network are reimagined versions of Robot Masters from the classic series. Then comes MegaMan Battle Network 4, whose Final Boss is an alternate NetNavi version of Duo from Mega Man 8... who logically shouldn't be any different, given that he came from space in both timelines. Later games seemed to realize this, as future NetNavis were less inspired by Classic-series bosses (although it did start looking more to the Mega Man X series at this point, particularly the duo of Colonel and Iris), and Sequel Series Mega Man Star Force completely abandons the idea of reimagining characters from the original timeline in part because of its heavy focus on characters coming from space.
- Misfile has an example that starts out being completely impenetrable, though the connection is eventually revealed: When main character Ash wakes up one morning having been retroactively turned into a girl, not only is he now female, as far as most of the universe is concerned he always was, with resulting changes in his wardrobe, photo albums, relationships... and, strangely enough, the half-completed car he'd stashed in his garage because he couldn't afford an engine for it suddenly has exactly the engine it needs. Eventually it's revealed that his estranged mother — whom he'd lost touch with in original timeline but had already reconnected with as a girl — had not only provided her with tons of clothes, she'd also bought her "daughter" a new engine for her racecar, perhaps to assuage the guilt she felt over abandoning her in childhood.
- SCP Foundation: Played for Drama and Horror with SCP-2003, a time machine designed by the Foundation to find out about possible futures and guide humanity onto the correct path. The problem is, not only are most of the futures they've seen quite horrifying, they've also found some people have a disproportionate effect on causality, even for events they're not part of. For example, the birth of a boy in Turkmenistan and the election of a New Zealand Prime Minister both happening on the same day in 2049 lead to a civilization-ending nuclear exchange between Israel and Greater Indonesia in 2058. All attempts to intervene with these two in any way only leads to the war happening sooner. The Foundation has a database on these and similar individuals.
- In the American Dad! episode "The Unincludeds", Steve's various attempts to make Snot popular turn Snot's future self into various bizarre alternates, ranging from a homeless guy, to a prisoner who is blind in one eye, to a trans woman, to a half-human half-turtle mutant, and lastly just a giant turtle.
- In the Ben 10 (2016) episode "Ben Again and Again", Billy Billions attempts to alter time so that he claimed the Omnitrix instead of Ben. When Ben and Gwen follow him, Ben tries to fix the timeline by getting the Omnitrix himself instead of letting his past self get it, but when they try to leave through the wormhole that they entered through, they just end up back in the past as bizarre alternate selves. They appear in medieval gear, then as cyborgs, and as cyborg pirates before Gwen reasons that the Omnitrix is intended for Past Ben and not Present Ben.
- The Bunsen Is a Beast episode "Beastern Standard Time" has Bunsen and Mikey go back in time to the stone age when their intentions were to go back to just before they were tardy so that Bunsen isn't late for school for a third time and able to enjoy Donut Day. Before returning to the present, Mikey gives the donut he was given to a caveboy that becomes interested in it. The pair later find that giving the caveboy the donut has resulted in Muckledunk becoming a donut-themed town called Donut Dunk, plus Amanda Killman is ruler of the town. Horrified by this change, Mikey and Bunsen prevent this timeline from happening by going back to the prehistoric era and replacing the donut given to the caveboy with an apple.
- In one episode of Cow and Chicken, they go to the arcade and get on a ride that takes them back to the dawn of creation. Cow drops a quarter in the primordial soup, resulting in mankind having quarters for heads when they get back to the present.
- The Fairly OddParents!:
- In "Father Time", Timmy went back to help his Dad win the trophy he accidentally melted. He comes back to find that the Internet is called "the Timmy" because Cosmo told someone to call it that. Somehow, this also resulted in "Internet" being Timmy's name.
- In "It's a Wishful Life", it turns out that if Timmy had never been born, AJ would have hair, Elmer wouldn't have a boil and a non-braceface Chester would have Timmy's fairy godparents. It turns out to actually be a test by Jorgen to see if Timmy was selfish enough to wish himself back after seeing this.
- In "I Dream of Cosmo", Timmy's Dad asks Cosmo, who was suffering from amnesia and thinking he was a genie, to wish his neighbor Dinkleberg never existed. Despite Cosmo's objection that it could result in this trope, he goes through with it, and they enter a post-apocalyptic world where somehow Timmy's Mom is a two-headed dragon.
- The Grim Adventures of Billy & Mandy: In Billy and Mandy's Big Boogie Adventure, the Bad Future versions of Billy and Irwin go back in time to stop Mandy from taking over Endsville. They succeed... only to find that Fred Fredburger is now in her position.
- On Family Guy, Peter went back in time to relive his teenage years and as a result, Lois ends up married to Quagmire and Peter is married to Molly Ringwald. Also, Al Gore is the President, we have universal health care, no crime or poverty, non-polluting flying cars that run on vegetable oil, and Dick Cheney, Antonin Scalia, Karl Rove and Tucker Carlson are all dead. But worst of all, Chevy Chase is hosting The Tonight Show. Peter manages to fix everything, but Roger Smith is now a member of the Griffin household.
- In the Freakazoid! episode "Freakazoid is History," Freakazoid is accidentally sent back in time to Pearl Harbor. He averts Japan's surprise attack and returns to the present. Much of the world seems the same, but Rush Limbaugh is a bleeding heart liberal, Sharon Stone can act, no Chevy Chase movies exist, cold fusion works, and Brain is the president (with Pinky as the Air Force One pilot).
- Inverted in the Futurama episode "The Late Philip J. Fry". Professor Farnsworth assassinates Hitler in the early 1930s in one timeline and accidentally assassinates Eleanor Roosevelt in another, but in both cases the year 3010 they return to appears to be exactly the same, except for being ten feet lower and five feet to the right.
- Downplayed in the My Little Pony: Friendship Is Magic episodes "The Cutie Re-Mark", Part 1 and Part 2. Starlight Glimmer travels back in time and prevents Rainbow Dash from performing her first Sonic Rainboom, which prevents the Mane Six from becoming friends — and allows several villains to conquer Equestria since the Mane Six were the ones to defeat those villains in the first place. That makes sense. Where it gets weird is when changes in how Starlight prevented that first Sonic Rainboom somehow result in a completely different villain triumphing each time, and wildly different alternate timelines as a result. So when Starlight casts a freezing spell on young Rainbow Dash, that causes a timeline where Equestria is in a drawn-out war with King Sombra and the Crystal Empire. But when Starlight talks Rainbow out of holding the race in the first place, that creates a timeline where changelings have overrun Equestria. And so on.
- A Robot Chicken sketch parodies Terminator by having the Terminator invoke this with random actions in the past that somehow alter the circumstances of the Resistance base as they prepare to send back their own agent. Among other changes are John Connor becoming Juan Carter, then Juanita Carter, and general chaos breaking loose as objects and people in the base transform randomly, such as becoming a herd of raptors hunting ballroom dancers before they all turn into butterflies.
- Many of the timelines seen in The Simpsons episode "Treehouse of Horror V" (e.g. donut rain as a result of killing dinosaurs). The story ends with everything normal, except for people having long, forked, prehensile tongues.
- In Teen Titans Go!, Cyborg and Beast Boy find themselves stuck 30 years in the future after having a staring contest to decide who gets the last slice of a pizza. Eventually, they take a time machine back to the present and decide to simply divide the slice between them. This leads to a future where humanity has been enslaved by robots.
- In the T.U.F.F. Puppy episode "Watch Dog", Dudley uses a time travel watch to go back and beat Kitty to the last donut in the break room, but he instead ends up in a reality where Snaptrap has taken over the world. At the end, Dudley uses it again when he misses the ice cream truck and successfully goes back in time, but changes the present so that he's wearing pants (which he hates).