Wrong Time Travel Savvy

"Martha Jones: But are we safe? I mean, can we move around and stuff?

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?"

- Doctor Who, "The Shakespeare Code"

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.

You Already Changed the Past can be an instance of this, if the characters in question are mistakenly confident that they could Set Right What Once Went Wrong (as opposed to just uncertain).

Comic Books

 * Universal War One: Kalish enumerates the different ways how time travel can work, and eliminate one because he thought it was impossible. But he was wrong, and

Film

 * 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 The Sarah Connor Chronicles (though that's okay; T3/Salvation and SCC are separate direct continuations from T2.) Chronicles actually has ; evidently time can be changed, but it's got a hell of a lot of inertia.
 * Arguably, in The Terminator, Kyle and Skynet erroneously believe history can be changed. Then Terminator 2 retcons it so that they're right.
 * In the new Star Trek movie, old Spock relies on Kirk's lack of knowledge about the mechanics of time travel in convincing Kirk not to reveal old Spock's existence to his past self.
 * 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.

Literature

 * Larry Niven's Get A Horse (and the other stories in the same series) 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 the hero 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, the hero of the Larry Niven time travel series, 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.
 * 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 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.
 * 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.
 * 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.

Live-Action TV

 * 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.
 * 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 Life On Mars and Ashes to Ashes, 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 A2A it all goes to hell when the Big Bad . Never the Selves Shall Meet indeed.
 * 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. But this fits with Carter's regular habit of introducing unnecessary complications into other people's plans just because the writers think it makes her look smarter.

Webcomics

 * Subverted in Homestuck. Dave has no idea how his time traveling powers work and doesn't plan on changing that.
 * Appears briefly in Schlock Mercenary when time travel suddenly becomes possible. Kevin resolves the matter experimentally, genuinely frightening Petey, the local Physical God.

Western Animation

 * Wakfu gives us, whose goal is to Set Right What Once Went Wrong.
 * 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..