The Doctor: Of course we can. Why not?
Martha Jones: It's like in those films: If you step on a butterfly, you change the future of the human race.
The Doctor: Then, don't step on any butterflies. What have butterflies ever done to you?
A key element in a lot of stories with Time Travel is the character's knowledge, or lack thereof, of how time travel works. Is the world deterministic? Is time hard to change, or way too easy? Is there something else weird going on? Will a Temporal Paradox destroy the universe, create an alternate one, or do you just risk being very confused? Of course, if the series itself keeps changing the rules it's up to the writer at the time whether or not you are wrong.
This trope occurs when the plot hinges on the fact that the characters think they know how time travel works in their world, but they are mistaken. Because time travel narratives often imply what events "must" occur, this trope can be a Necessary Weasel so as to maintain surprise (including surprise that the foreseen events do in fact occur).
- In Dragon Ball Super Present Zamasu does this when Beerus, Goku, and Whis confront him about his plan to murder Gowasu and steal Goku's body (though at this point they think it's his plan to murder Gowasu and create an Evil Knockoff of Goku). Since they know about it from confronting his future self who succeeded in this plan, he thinks that means his victory is inevitable. Actually, time travel in Dragon Ball runs (for the most part) on multiverse theory, meaning changes to the past just split timelines. Beerus promptly disintegrates him, creating a new timeline in which he died before he could accomplish anything. Beerus then commits this trope when he assumes that Zamasu's future self was erased, when in actuality he was protected from the change via the time ring he wore.
- Based on the idea of travelling to alternate universes being a different form of time travel, this essentially applies in Cross Cases when Harry Dresden initially learns that new acquaintance Sam Winchester is from an alternate universe. Harry assumes that "alternate universe" just refers to the kind of universe where a specific part of history changed, such as a world where Harry Dresden was never born, but everything else he understands about the world still applies. However, he is later informed that "alternate universe" could involve a divergence taking place so far back in the past that the very rules of magic have been changed as well, with the result that Dresden can't be certain Sam's opponents will play by the rules he relies on to keep himself ahead of his enemies.
- 12 Monkeys starts with the main characters seeming to know how time travel works, but by the end it's a bit more ambiguous whether they do or not. This is probably because the main character starts to feel like he's losing his mind and possibly dreaming up the whole future nonsense in his own mind.
- The main characters of Primer start out by taking elaborate precautions to avoid changing the timeline too much. By the end of the movie, they've realized that they're in a branching universe-type situation where they can totally ignore these precautions. By then, they've already spun the timeline wildly out of control.
- In Star Trek (2009), old Spock claims Never the Selves Shall Meet to convince Kirk not to reveal his existence to his past self. He's actually lying, and just wanted to keep away from them so their friendship would develop naturally, but Kirk had no way to know that.
- One interpretation of Terminator Salvation (and the whole series) is that the characters are tragically wrong about the whole "no fate" thing.
- That doesn't fit with Terminator: The Sarah Connor Chronicles (though that's okay; T3/Salvation and SCC are separate direct continuations from T2.) Chronicles actually has different time travelers bringing back knowledge of different futures, though all feature the Skynet War; evidently time can be changed, but it's got a hell of a lot of inertia. It gets even more complicated in SCC when we find out there are actually three factions trying to alter history, not just two. In addition to the Human Resistance and Skynet, there's also a group of rogue Terminators with their own mysterious agenda. When all of three groups mess with each other's attempts to change the past, it's a real Mind Screw to even guess at who's "winning" and which version of the future we're currently on a path toward.
- Arguably, in The Terminator, Kyle and Skynet erroneously believe history can be changed. Then Terminator 2: Judgment Day retcons it so that they're right. note
- Avengers: Endgame: Scott Lang and Rhodey expect time travel to work just like in Back to the Future and almost every other movie dealing with the subject. A frustrated Hulk explicitly tells them that time travel doesn't work that way. For instance, Scott believes in Never the Selves Shall Meet, while present Steve ends up in a Mirror Match with the Steve from just after the Battle of New York.
- Interestingly enough, on Runaways, which also takes place in the MCU, time travel works exactly the way Scott and Rhodey think it does.
- In Dinoverse this once comes up. Four children have accidentally had their minds converted into "pure thought-energy brainwaves" and sent to the Cretaceous into the bodies of large prehistoric reptiles; as they're orienting themselves and wondering what happened, they get a message from the distant future, from sixty years after the accident. The message claims that they've been comatose for all sixty years but then posits a way they might get back and to an instant after the accident. The characters worry about this off and on, wondering if it means they've already failed, or if there's some paradox making it possible, or if they'll be dictating that message in the future.
- Downtiming the Nightside basically runs on this. As the story begins, time travel is brand-new and the people working with it are gradually piecing the rules together as they go along. There's a lot they miss, as protagonist Ron Moosic learns when he starts running into people from the future, who fill in several of the gaps. And then it turns out they don't really understand it either and are doing the same thing, just with more experience to work from. The understood rules of time travel are constantly twisted and broken, but the truth is that nobody truly knows what the rules actually are , which leads to, among other things, a truly epic instance of Screw Yourself.
- Alfred Bester's The Men Who Murdered Mohammed plays around with this. A scientist attempts to erase his wife out of existence after he finds her cheating on him. He whips up a time machine, and goes back in time to kill her grandfather. The catch? It doesn't work. So, he works bigger, rampaging through time, killing more and more famous people (hence the title, as even when he murders the founder of a major world religion nothing changes) with absolutely no effect on the present until, finally, he meets a fellow time traveler who explains that the past he's killing is his own, and he's unhinged himself from reality because of his actions.
- Larry Niven: The Hanville Svetz series (beginning with "Get a Horse") features time travel based on the premise that, since time travel is impossible, if you travel back in time you actually enter a fantasy world. Thus, when Svetz goes back in time to bring back a horse, he finds a unicorn. When he goes back to bring back a whale, he finds Moby Dick, and so on. No one in the series ever figures out that they aren't visiting the past, but rather are visiting fiction. Svetz also finds a werewolf when he is after a dog, and finds a Roc chick when going after an ostrich. In Rainbow Mars, the pattern continues, as the ancient Mars the characters visit contains elements from fictional Marses created by Edgar Rice Burroughs, Ray Bradbury, H. G. Wells and others.
- The Time Ships by Stephen Baxter is a sequel to H. G. Wells' The Time Machine, detailing the Time Traveler's adventures after he vanishes at the end of the first book. He's heading back to the future to rescue Weena (a rare instance of Set Right What Once Went Wrong in which the "once" is in the future, though in his personal past), but finds the future radically changed because his story was published as a novel, preventing the Bad Future of Eloi and Morlocks. After learning this he muses dismally about how he'd expected history to be like a room he could move through while it remained basically unchanged.
- Connie Willis' works, particularly To Say Nothing of the Dog and Blackout. The protagonists spend a lot of time worrying about the Butterfly Effect and have a fairly well-developed theory of Rubber-Band History... but in every case it seems to work out as You Already Changed the Past.
- Doctor Who has had several examples during its long run — it doesn't hurt that the series itself doesn't stick to any hard rules about what you can and can't change. The Doctor often has to tell companions that they can't rely on Foregone Conclusion when facing threats in their relative pasts.
- Expanded media has generally elaborated on the rules to explain that, while there are fixed points in time that absolutely cannot be changed, history itself is malleable enough that time travellers can get involved in events and reality will adjust to accommodate their presence so long as they don't actively interfere with anything they know happened a certain way.
- In Life On Mars and Ashes to Ashes (2008), both protagonists attempt to change history, but fail despite their best efforts, leading them to draw the conclusion that the past cannot be changed. They also meet close relations and have close encounters with, but deliberately avoid actually meeting themselves. Then suddenly in series 2 of Ashes, it all goes to hell when the Big Bad shoots his younger self in the face and frames Alex for the murder. Never the Selves Shall Meet indeed.
- A plot point on Lost. Faraday spent the first block of episodes of the season convincing the other characters (and viewers) that the past cannot be changed. Just when everyone was starting to get it, Faraday changes his mind and the characters ended the season trying to dramatically alter their own destinies.
- His initial statements were correct: You can't change the future. Time is immutable. If you go back in time, you are already a part of the past that led to the present you came from, and you always have been (or will be). As Miles explains to Hugo, Back to the Future was completely absurd. Either you participate in the past, directly or indirectly causing your future, or you just stand aside and let things happen. Actually, since whatever happened happened, it doesn't matter which you chose because, from your perspective as someone from the future, you already made the choice. And you can't even create an alternate realities. That so-called AU in season six was really just a shared post-mortem hallucination. The fact that it looked like an alternate timeline was a complete red herring.
- Faraday attempts to change the past, and apparently manages it once with Desmond, and Desmond apparently does change the past a few times...but both of them just coincidentally manage to have 'swiss cheese memories', the existence of which can be linked to their time travel, so smart money said they already changed the past the whole time, and the way causality protects itself on Lost is to simply make people remember things wrong, or not at all, if the memories would cause a paradox.
- In the Stargate SG-1 episode "1969", Carter spends most of the episode telling the rest of the team they have to be careful not to change the past. Yet she doesn't seem to realize that Hammond's handing them the note before they left pretty much proves they're in a Stable Time Loop. On the other hand, later episodes dealing with Time Travel reveal that history can be changed.
- Several different forms of time travel exist in Final Fantasy XIV, each with their own set of rules and inconsistencies. One that is proposed by a character in Endwalker is a form of Astral Projection, allowing the user to see and hear the world of the past but not interact or alter events. A character would eventually put this into action, and it turns out the proposal missed one small detail: Elidibus only factored in how much power the Crystal Tower would require to send the Warrior of Light to the past. Once they arrive, they have just enough of a presence that Hythlodaeus and Emet-Selch can both sense them and use their own magic to give them form. Welcome to The World Unsundered! The present is still unalterable but there are more than a few loopholes now.
- In Frankie and Stein Shelly is quite wrong when she insists that meeting the future versions of themselves would cause her and Stein's heads to explode.
- Subverted in Homestuck. Dave has no idea how his time traveling powers work and doesn't plan on changing that.
- Played with Terezi, who encourages John to fight his Denizen, because she has a computer program that can see into the future and knows that John survives well after the time of the fight. Naturally, John dies and creates an alternate, doomed timeline that Dave has to fix. Terezi is much more careful about this from then on.
- Appears briefly in Schlock Mercenary when time travel suddenly becomes possible. Kevin resolves the matter experimentally, genuinely frightening Petey, the local Physical God.
- In one of the final arcs of Umlaut House, and the second arc of Umlaut House 2, Pierce is concerned that Rhonda's time travel could cause him to cease to exist. Instead he realizes that the timeline is fixed when his father asks him to think of a number between 1 and 16, then asks how old he was when he spontaneously decided to repaint the kitchen (they were the same number).
- Dinobot in Beast Wars fears that You Already Changed the Past is in effect, rendering all of his choices meaningless. Then Megatron proves that you can change the past and watch the future change with it, which drives the plot from that point on.
- In the episode "Paraducks", Gosalyn advises Darkwing Duck to not interfere when they get sent back in time to his childhood. Back to the present, and the city's held in the grip of a crime lord. Turns out they were in the middle of a Stable Time Loop instead.
- In the Justice League Unlimited episode "The Once and Future Thing: Time, Warped", Static concludes that the heroes' victory is a Foregone Conclusion because Bruce Wayne is still alive in the future. Bruce explains that it's not that simple — he has no memory of traveling into the future during his younger days as Batman, which means that the timeline is not definitively fixed.
- Wakfu: Nox wants to set the universe back two centuries to save his family by amplifying existing time-control magic as much as possible, using whatever means necessary. Grougaloragran says this is likely to just destroy the universe, but Nox is very sure of himself and unconcerned with the consequences of being wrong. It turns out Nox was closer to the truth, but not by much. He really could turn back time without damaging the universe, but even with the amount of wakfu he gathered over 200 years of research and genocide (up to and including draining the source of life for every Sadida on the planet) not counting what he ended up wasting over the course of his battles, he could only manage to go back in time by twenty minutes; going back two centuries would have required over five million times as much power as he had gathered.
- Xiaolin Showdown: Omi freezes himself to recover the Sands of Time from his future self. Spot the flaw in this plan. If Omi spends all of his time frozen, he can't give himself the Sands of Time because his old self wouldn't exist. The show does point this out.
- In Futurama's "Roswell That Ends Well," Farnsworth has no idea how in-universe time travel works since it's an uncommon event that can only happen through a freak accident. It turns out the past is unalterable in this case, though the series as a whole made it more variable.
Farnsworth: You mustn't interfere with the past. Don't do anything that affects anything, unless it turns out you were supposed to do it. In which case, for the love of God, don't not do it!
- In DuckTales (2017), Louie hatches a Get-Rich-Quick Scheme where he uses Gyro's time travel tub to go into the past and bring back valuable treasures. To ensure he doesn't screw up the time stream, he makes sure to only take artifacts that were lost to history moments before they were recorded to be lost. Unfortunately, he learns the hard way that in the show's universe, taking any object from it's proper place in time disrupts the time stream and nearly causes time and space to collapse.