A contradiction of causality within the timeline brought about by Time Travel. Theorized to be dangerous to the fabric of reality, and known to be dangerous to the brains of anyone who tries to get their head around them. So don't. It's usually what The Professor worries about during a Time Travel story.
Punishments for creating a paradox vary. You might instantly vanish from history or cause your time-travelling self to be erased; you might be immune but find the world around you different; you might destroy reality itself; heck, you might even accidentally unleashkiller flying time monkeys.
This is all fictional, of course. In reality, a confirmed paradox would disallow time travel to work at all. You see, a logical paradox is not a thing. It is a sign in a human-created model that either you have attempted something impossible or that you have incomplete understanding of how something works. Here's the thing: all our notions of causality are based on the fact that time only moves in the one direction and a paradox exploits this cause and effect relationship. Once you throw Time Travel into the equation and have time to move in a different way, it's really anyone's guess what will happen. Not that this prevents authors from abusing the concept as a sort of universe-wide Logic Bomb.
There are many kinds of paradoxes that can be created by poorly thought-out time travel, but it usually fits one of these two major categories:
This paradox gets its name for a very simple question: "what would happen to you if you went back in time and killed your grandfather before he had offspring?" (Parodied by Futurama; see example in Stable Time Loop)
More generally, this means doing something that makes your time travel impossible or unnecessary. For instance, if success in the time travel endeavor means that the condition you set out to change never happens, then you won't ever have had any reason to come back and try to change it. Thus, without your intervention, it will happen after all, meaning you then come back to change it, meaning you don't have to, meaning you have to, and so on, and so on, etc.
While it can destroy the universe if the author is cruel, nowadays it's more likely to fall into the domain of the multiverse theory: According to this theory there are an infinite number of universes, differentiated by things ranging from a single misplaced atom to inexistence of life. According to this theory if you travel back in time what you really do is travel to the present day of an Alternate Universe just like your own, but which got started later. In this scenario, you can make anything happen and not trigger a paradox. Complicated, isn't it?
Note that this kind of paradox is deemed irrelevant in most cases of Mental Time Travel.
The kind of paradox that occurs in a Stable Time Loop if you're not very careful about what you're doing, involving events that are their own causes. While not a paradox in the strictest sense - events remain self-consistent - it does violate normal expectations in surprising waysnote Quantum physics comes into play, see discussion. There are several variations:
The Object Loop: When an object from the future is sent into the past, takes The Slow Path back to the future, and then gets sent back into the past again, in the same way, for the same purpose. For example you travel to the past and sell a pair of antique glasses you got from a friend, who it turns out got them from his grandfather, who bought them from the time traveler. Where do the glasses come from in the first place? The glasses have literally come from nothing. This is sometimes called a "closed timelike curve" in hard SF.
You might ask how the object escapes erosion or other damage too? Since its origin point is also its end point the object cannot (relative to itself) loop endlessly as it would experience infinite decay (relative to itself) and thus would not exist to be sent to the past. Preventing the loop in the first place. Thus to exist the object must (improbably) escape all forms of damage/erosion/entropy between its arrival and departure. Which is technically not impossible but only deepens the weirdness of such an object existing.
In one variation, information rather than a physical object comes out of nowhere: for instance, an engineer from the future giving the formula for transparent aluminum to the historical "inventor". This version avoids the "erosion" issue, provided that the information is correctly transmitted and recorded. The actual effect of this variation is that no one invented the object, or no one discovered the formula/concept. It's just there.
The Reverse Grandfather Paradox: When time travel is involved, cause and effect tend to get muddled. Say you remember being involved in an accident as a child, and would have died if not for the intervention of a mysterious stranger who showed up, saved your life and then vanished without a trace. Then you become a time traveler and find yourself at the scene of the accident, and there's a little kid who needs saving. That's right: you happen to be the mysterious rescuer. Instead of accidentally making your time travel unnecessary or impossible by meddling with the past, your meddling somehow made it possible in the first place. But then the question becomes how you "originally" (technically meaningless in this context) survived to time-travel and save yourself— and thus the "paradox", which is not actually a paradox in the logical sense, but a confusing and counter-intuitive result of time travel. This also precludes a Multiverse explanation, since both child and rescuer-adult occupy the same timeline and universe, if the child has a childhood-memory of being rescued by the adult-self.
Normally, as written, the temporal paradox never turns out to be as dangerous as The Professor imagined it would be, or it turns out the characters were "supposed to do it"in the original timeline. The latter ontological paradox is also known as a predestination paradox, and the resultant philosophical questions are rarely thought about in the series.
If two time periods are featured, the effects of a paradox will usually be visible in the future only "after" the cause has happened in the past (see Meanwhile, in the Future).
Interestingly, series rarely have the same result to paradoxes even in the same show.
The most common effect of a paradox, on TV at least, is to trigger the Reset Button and unmake the entire episode's consequences.
Theoretically, a paradox that consists of two mutually-exclusive events can have one of two results: either the fabric of reality rips itself apart trying to determine which reality is the 'correct' one, or — according to Multiverse Theory — it's discovered that causing a paradox is a technical impossibility, as each supposed 'paradox' merely creates two 'alternate' timelines — one for 'Situation A' and another for 'Situation B'.
(Of course, Multiverse Theory also holds that time-travel is hypothetically possible — since every choice made, and every action taken, and every word ever written, creates a series of 'alternate universes', each being slightly different to account for the results of the choice/action/word, we would just need a consistent way to travel 'between' the various 'multiverses' thus created.) On the flip side, it also means that it's impossible to completelySet Right What Once Went Wrong, as the "original" timeline will always be unchanged.
Compare Timey-Wimey Ball, Stable Time Loop. If time is somehow dangerous besides from paradox, it's Time Is Dangerous.
Grandfather paradox examples:
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Anime and Manga
Akira Toriyama follows the multiverse approach in Dragon Ball Z: Future Trunks travels back in time to change the past, although he knows that this will not affect his own past or future (since each timeline exists as a separate dimension, changes made in one time line will not affect the others). Because Cell was also traveling around, but then Trunks prevents Cell's trip, fans have theorized that there are at least five time lines: the time line chronicled by the anime and manga (Line 1), the time line Trunks-who-participates-in-the-Cell-Games (this is Future Trunks, mentioned above) originates from (Line 2), and the time line Cell-who-killed-Trunks-to-steal-the-time-machine-and-terrorized-the-main-series-time-line originates from (Line 3), plus two theoretical/implied time lines. The first implied time line is the one visited by the Trunks native to Line 3, the Trunks killed by Cell so that he could go back to Line 1. This time line would proceed identically as Line 1 up to the point of Cell's discovery; after that, all fans have been able to guess is that the androids were somehow defeated. The commonly-accepted (suggested by the Daizenshuu guidebooks) theory is that Bulma built the deactivation switch (just like she did in Line 1) but Kuririn didn't destroy it, and Trunks took it back to Line 3 to use on the androids there. (Of course, it was the very discovery of Cell that lead Trunks and Kuririn back to Dr. Gero's lab in order to get the blueprints Bulma used to build the switch... so maybe Piccolo defeated them...?) The other theoretical time line is one Cell would have visited, if the Trunks native to Line 2 didn't kill him, using the strength gained from his training in Line 1. This line would proceed identically to Line 1 up until Cell's discovery, as well, so the Trunks visiting this time line probably won't be strong enough to defeat Cell, if he comes from a time line where Cell is waiting to kill him if he kills the androids and...
Haruhi Suzumiya: Every instance of time travel in the stories (and there are many) invariably generates paradoxes like these. Characters go back in time to save themselves, information comes out of nowhere, etc., etc. Of course, no explanation is ever given in the books.
The Marvel Universe has a simple solution for this in the novel trilogy Time's Arrow. There are a large—but not infinite—number of alternate universes, that deal with what ifs. If someone in those timelines goes back in time to change something, it will create a new timeline that's an offshoot of one's own from that point. No going back and killing Hitler, Cyclops notes when told this—the idea being that if you do so, your own timeline will be unaffected. Oddly, this doesn't seem to be the case in the comic universe.
Except when it is that way. You don't think that any two comic writers actually agree on how this stuff works, do you? That said, the Earth X series (including Universe X and Paradise X) suggests a couple of different versions of this. In the end, it is fundamentally, philosophically important that the idea that alternate universes branch off only as a result of time travel is true.
This is generally accepted; however, it has been shown that Doctor Doom has invented technology that allows this rule to be broken in PAD's X-Factor run.
This is the reason time traveling villain Kang the Conqueror keeps multiplying, often despite his own wishes.
One run of Thunderbolts ended with the present-Thunderbolts meeting the past versions of themselves. Fixer killed his own past self, and the universe promptly began collapsing. Fixer resolved it by having himself de-aged and his memories erased, in order to replace himself in the past. He thereby condemned himself to live in an infinite loop, reliving the same period of time over and over for eternity.
One exception, however, are the Space Phantoms, servants of time-traveling The Avengers villain Immortus, who learned the hard way how dangerous this sort of thing is. Apparently, the Phantoms learned time travel before they learned space travel, and when a civil war broke out between them, each side tried using time travel to change the outcomes of important battles. If a side did so successfully, the other side would try to undo it, again and again, until finally, the constant meddling with the temporal flow destroyed their world, leaving them a Dying Race trapped in the dimension of Limbo who are little more than slaves to Immortus' will.
Grant Morrison's legendarily complex and metafictional series The Invisibles hinges itself on contradiction, and details several brainbending temporal paradoxes. It would perhaps be remiss to go into any of them here in any great detail. Basically, if you like that sort of thing, go read The Invisibles.
However, the effects of time travel are different in the various movies. For instance: in the first movie, Marty's accidental stopping his parents getting together was starting to delete him from existence; in the second movie, Biff interferes with his past, and the changed present has Biff saying he sent Marty to a school in Switzerland, yet this never affects the time-travelling Marty.
Fanon has justified this in various ways; for example, saying the time traveller is only affected by his own changes to the timeline, or by saying that he will be unaffected as long as there's somewhere in the timeline for him to "slot in" - changes to his situation in the new timeline are shrugged off, as long as he exists somewhere.
The documentaries on the DVD set mention how the justification was that there's some entity that regulates time itself. The partial deletion over time of Marty, why both Jennifers fainted when they met each other, and why even with relatively major changes to the timeline, Marty's family, home, and association with Doc Brown and Jennifer remain largely the same. They wanted to explore this aspect, but couldn't find a way to incorporate it into the films without it being obtrusive. The Other Wiki has more information here.
One possibility is that there are two Martys, just as there were in 2015 and when he returned to 1955.
Much stranger is how Biff, when he returned to 2015, returns to the version where he is a loser, as opposed to the one where he is rich. The only clear reason for this was so that Marty and Doc could get the DeLorean back.
The Lake House is a story about a mailbox that delivers letters from Kate to Alex two years ago and vice versa. Alex dies in a car accident on Valentines Day. Two years later, when Kate realizes that, she sends a message to Alex two years ago telling him not to be there, and he survives. It should be noted that the Korean movie this movie is based on dealt with the paradox differently: The female character sends the warning back in time, but the male character remains dead. Meanwhile, the insertion of the warning splits off an alternate universe where the male character survives, and the movie ends with the male character meeting the female character, just as the female character is moving into the house, before she's even gotten the first letter. It's okay, though. The guy has quite a story to tell her. Since the movie ends there, by the way, it's unknown whether the female character would have ever started the letter-exchanging if the guy hadn't...ugh, it's all sort of vague, really.
The horror film Triangle has loads upon loads. How it works, nobody knows, as even Phelous can tell you.
Frequency depicts basically the same situation as The Lake House—due to abnormal sunspot activity, a police officer and his long-deceased father are able to communicate across a 30-year gulf of time over the same CB radio set. The son first saves his father from dying in a firefighting mishap, only to discover that he died of lung cancer a few years later anyway. But he then manages to convince him to quit smoking.
Donnie Darko: After sleepwalking away from the place where he was supposed to die, the eponymous character is caught in an unstable time loop that he must close. When he moves himself and the jet engine that should have killed him back into the past, he closes the loop by dying in the way that he should have from the beginning, negating everything in the time loop. This causes everything that was changed by his time travel to exist outside of the normal timeline without affecting it. Maybe.
The whole plot of the movie The Butterfly Effect revolves around the main character's ability to travel back in time and change parts of his life. Every change causes his brain to physically rewire itself with the new memories, though, and this causes intense pain for him.
Meet the Robinsons. Let's see. If Goob made the catch and won, getting himself adopted and never becoming the Bowler Hat Guy, Lewis would never have learned that Goob became that person, and never bothered to prevent it. Yeah. And he wouldn't known not to create Doris.
Averted in The Time Machine (2002), where the time traveler attempts to save his fiancee, but she always dies on the same night no matter what he does. He travels into the future to find out why. In the far, far future, he learns from a more evolved human that if he saved her he would not have the motivation to build his time machine. Apparently, a paradox is allowed if it doesn't prevent the time travel device's creation, though.
In Wishmaster, the main character gets one wish from the evil djinn, after which it will be freed to work its evil upon the world unfettered. She wishes that a particular forklift operator had not been drinking on the job a few days before. Since he wasn't drinking on the job, he didn't drop a particular crate. Since he didn't drop the crate, the statue inside never broke. Since the statue never broke, the gem inside was never uncovered. And since the gem inside was never uncovered, the djinn trapped inside it was never released. But wait! Since the djinn inside it was never released, that means he never granted the protagonist's wish! And if he never granted the wish, then he was released after all! But then...
Lazarus Long, protagonist of Robert A. Heinlein's Time Enough for Love creates a time machine and argues that it would not be possible for him to change the past, because in doing so he would also change the future—in the essence, negating his own existence, or at least the details of it—and making his own journey into the past improbable at best, if not impossible.
David Weber's The Apocalypse Troll has the characters discussing the theories about time travel — one (it's not possible) has been disproved by the fact that one character just did, to arrive in the time of the discussion; the other two, that the future will be altered by what she did or that her presence has caused an alternate world to split off, can't be proved or disproved by anything they can do now. They end up assuming the alternate world and thereafter ignore the question.
The Time Scout novels avoid Temporal Paradox by the timeline including built-in safeguards; safeguards which are dangerous to time travelers. The most prominent are first, that you can't change anything that's important to the timeline—some improbable accident will occur to prevent it, no matter what you try—which is dangerous, as although some people, objects and events are obviously important to the timeline, there are even more that aren't obviously important, but just as crucial; and second, that if a time traveler ever arrives at a time where they already exist, the most recent version dies instantly to prevent them from doing anything to their past selves that would undermine their current presence.
To say nothing about what happens to Jake in the first part of The Waste Lands, who is both alive and dead at the same time.
The Caretaker Trilogy has an interesting take on this: there are no alternate universes, and while changing the future/past is possible, doing anything that would create a paradox is impossible simply because it would create a paradox. It's said that there is some natural "force" that prevents paradoxes from occurring. Exactly how that works is not explained, because the protagonist apparently doesn't have the necessary education to understand the specifics.
Time travel is forbidden in The Dresden Files because it might end up destroying the fabric of reality. Characters capable of seeing the future can't be specific about their visions for the same reason.
The Gatekeeper, specifically, has a vision of something major in the Dresdenverse, and alerts Harry to it, in the most vague, roundabout way. Bob later explains he did this to avoid the entire universe going kaput. He also mentions that no one has ever caused a temporal paradox before, and you can tell by the way the universe keeps existing.
In The Anubis Gates by Tim Powers, main character Brendon Doyle, a modern expert on the poet William Ashbless, ends up back in the 1800's during Ashbless' lifetime. When Doyle ends up BECOMING Ashbless thanks to a body-snatching werewolf (don't ask), he publishes the poems from memory—which leaves us with the problem of how the poems were written in the first place. In fact, it actually freaks Doyle out, but he concludes that as long as the poems exist, history will continue in its proper order, so he shouldn't sweat too much over it.
Gregory Benford's Timescape describes a unique, quantum-mechanical approach to Grandfather Paradoxes. If a time-travelling signal were to prevent its own transmission, the signal and everything involved in triggering it would be in an indeterminate state where it neither does, nor doesn't, occur — like Schrödinger's Cat before the box is opened.
The Hitchhiker's Guide to the Galaxy universe is full of this, particularly in the third book. A correction-fluid manufacturer tries to get an endorsement from a tragic poet and ends up preventing the tragedy that inspired him. A landmark cathedral is torn down to make way for a refinery, but in order to open on time they had to start construction so far back in time that said cathedral was never built.
Worst of all are aorist rods, which provided power to the present by depleting the power reserves of the past ... when it was discovered those bastards in the future were doing the exact same thing, the rods and all knowledge of their manufacture was destroyed to stop what was already happening now from occurring in the future.
In Dragonlance, Raistlin kills Fistandantilus and usurps his soul, and then goes forth to succeed where Fistandantilus failed in traveling into the realm of the Gods. Since it was Fistandantilus' drifting soul that resulted from that first failure which saved Raistlin's life during his Test in the first place, I think we can all say that Raistlin pretty much screwed causality in the ear.
Live Action TV
In Quantum Leap, it appears that Sam is affected by the changes he makes to history only after he leaps, and this has some bearing on his occasional manifestation of previously unmentioned skills (and previously unmentioned/nonexistent family members). Al, on the other hand, seems to be affected instantly, but only when probability of a new event becomes sufficiently high. (In one episode, Sam assures Al's untimely death. When the probability reaches 100%, Al is replaced by another character, but he reappears when Sam reduces the probability.)
One has to give credit to Doctor Who, in that a show with a time traveler as a central character delves into temporal paradoxes relatively infrequently; in most cases, the time travelling is just a way to set stories in different periods, the temporal version of Adventure Towns. It does have its fair share of 'em though (especially after Steven Moffat started writing for the new series):
"The Time Meddler" has characters speculate that if history was changed, their memories would be updated with the new version instantly — though later events imply this is not actually the case.
In "Father's Day", we see that creating a true paradox (which seems to require not only a change to history which undermines the traveller's presence, but that the traveller witnesses himself doing this by being present in the same time zone twice) has the effect of releasing Clock Roaches, which eat everything on your planet. No, really. The earlier example was retconned in the later one with a Hand Wave by the Doctor saying that when the Time Lords were still alive they prevented this sort of thing from happening.
The series does tend to imply that the "Laws of Time" are more of a legal code than physical law: in "Smith and Jones", the Doctor notes that crossing one's own timeline is dangerous and forbidden, "except for cheap tricks."
Although there's the recurring concept of fixed events as opposed to unfixed ones — events that must happen in a specific way, as opposed to ones that could happen any way. The Doctor, of course, has the inherent ability to tell them apart. And, of course, no-one else does. Usually.
Also, in the old series Gallifrey had the Eye of Harmony, a modified black hole that acted as an unlimited power source, universe-wide navigational beacon, and the mother of all temporal stabilizers. Thus even if they screwed up, the Time Lords had access to enough energy to maintain the desired timeline by brute force if necessary (as seen in "The Five Doctors"). In the new series, the Eye of Harmony has been destroyed, so the Doctor has less to work with.
Another interesting use of the temporal paradox concept comes in "Last of the Time Lords", in which the Master brings humans back in time from the end of the universe to kill humanity... which would normally make no sense, which is why he turned the TARDIS into a "Paradox Machine" to keep the paradox stable. Destroying this acts as a Reset Button which sets everything on the surface back to the way it was before the machine was activated.
Also, "Blink", the episode that gave us the Timey-Wimey Ball, has a paradox at its heart. The Doctor is only able to tell Sally Sparrow what's going on via DVD Easter Eggs because Sally wrote it all down at the time and gives it to him at the end of the episode.
To make things more interesting, "The Time of Angels" reveals to us that the image of an Angel is an Angel and with everything in that folder she handed the Doctor, the transcript, several pictures of Angel statues, the list... we can wonder where those scavenger Weeping Angels came from anyway.
At one point, they infected the Third/Fourth Doctor with Faction biodata during a regeneration that wasn't supposed to happen (when he was shot on Dust, instead of the canon radiation poisoning on Metebelis Three), causing the Eighth Doctor to disrupt his own timeline so that the Third Doctor was shot on Dust, permitting the Faction to infect him with the biodata, which caused him to tinker with the past so he could be infected with the biodata... BOOM!
You really have to give credit to "A Christmas Carol" and how many paradoxes it goes through. Traveling back in someone's personal timeline as they watch from the future. Confusing and rather nonsensical; where're the Reapers in all this?! And then the Doctor brings the past version of Scrooge — er, Kazran, to visit his future self. Attempts to follow this seriously may lead to your head asploding.
But then the Grand Moff (who wrote "Blink", "Christmas Carol", and other Timey-Wimey eps like "The Big Bang" and "The Girl in the Fireplace") is becoming quite known for his confuddling paradoxes. I mean, look at the contributions to Comic Relief! (Both of which written by him.)
Lampshaded in "The Girl Who Waited". Amy is trapped in a faster timestream. Rory encounters an Amy who has waited 36 years to be rescued. She is the key to saving a younger Amy who has only waited for one week in the timestream, but saving the younger Amy means the older Amy would never have existed to save the younger Amy.
The Doctor tries to invoke this in "The Angels Take Manhattan". Since the Weeping Angels feed on time energy, creating a paradox would "poison the well" and kill the Angels off.
In The Big Bang Theory, in one episode, Sheldon, Leonard, Raj, and Walowitz buy the original time machine prop. Events in the episode lead to Sheldon and Leonard trying to decide if Leonard could have gone back in time to stop himself from buying the time machine, leading Sheldon to say, basically, "No."
LOST: Subverted when Sayid attempted to kill Ben, which simply caused him to grow up into the man he already was. Played straight with the Compass, which is a textbook example of the abovementioned Object Paradox: looping endlessly between 1954 and 2007.
Babylon 5 had a very weird one. During the Minbari War, the aforementioned aliens stopped short of destroying humanity when they discovered that a character held Valen's (Alien Jesus) soul, explaining where the souls of the Minbari had been going in the past centuries. They believed that Valen re-incarnated into that character. However, in a later episode, it is discovered that this character travels back in time and assumes the role of Valen. So, it's not that Valen's soul went into that man, but that this man was the original Valen soul. One has to wonder if there isn't a 2nd receptacle for Valen's soul.
This also falls into the ontological paradox category when you think about it for a bit. If the Minbari had chosen a different fighter from among the thousands at the Battle of the Line to capture, the triluminary would not have activated, and rather than sparing humanity they would have wiped them out. Which would mean that Sinclair would never have gone back in time, which would mean they never would have gotten the triluminary (or defeated the Shadows, probably), which would...yeah.
Furthermore, if Valen, who founded modern Minbari society and taught them their globally-accepted philosophy, actually learned everything he knows about Minbari society and philosophy from modern Minbari themselves, that creates an Information Loop: he is teaching them things he only knows because they taught him first.
In the 3rd series of Misfits future Simon paid £10,000 to past Seth for the immunity to Alisha's power. He got the money from...Seth in the future. Who gave it to Simon in the future, who gave it to Seth in the past, etc.
In the RPG Feng Shui there are no temporal paradoxes, because history rewrites itself to accommodate changes in the timeline. For instance, if Donald Fong goes back in time and kills his great-great-grandfather, when he returns to the present, he'll find that everyone now knows him as Donald Wong, a person with a very similar life to Donald Fong. He'll remember his old life as Donald Fong, but everyone else will always have known him as Donald Wong. In extreme cases - such as when someone controls enough feng shui sites to cause a critical shift (i.e. they change reality) - people can get written out of the timeline entirely; they still exist, but they have no past in the current timeline, because their version of history simply doesn't exist anymore.
The German RPG The Dark Eye takes a similar approach in declaring time a dynamic, "healing" weave. An example to solve the grandfather paradox is to have the person get stranded in time, get a life, meet a woman, marry and have kids and thus becoming his own grandfather.
In Achron, paradoxes are a deterministic and fully logical gameplay element.
The Metal Gear Solid games are well known for situations occurring in which the player can create a paradox of sorts, by killing someone in a prequel who is known to be alive in chronologically later games. Of particular note is Revolver Ocelot, in the third game, whose death during certain scenes results in an instant Non-Standard Game Over. It's especially surreal when your CO from the future starts chewing you out for causing a Time Paradox.
The "CO from the future" part is actually a joke that is, for lack of a better way of saying it, lost in translation. In the original Japanese language track, the voice actor who plays this character is also the voice actor who does the Japanese dub of Doc Brown in Back to the Future.
In the Space Quest series of games, Roger Wilco is saved from certain death in Space Quest IV by a mysterious man who is later revealed to be his grown-up son from the future. Roger meets the future mother of Roger Jr. (though she doesn't know it yet) in Space Quest V and if she's killed during the course of the story, Roger Jr. and therefore Roger Sr. as well cease to exist, and it's Game Over. However, in one way to kill off Beatrice picking her up while she's frozen and having her break into bite-sized pieces, you get a slightly different Have a Nice Death message.
In Tales of Phantasia, Time Travel divides the timeline around halfway through the game in such a way that the object of your characters' revenge and the final boss of the game are two different people. Not only that, but at the end of the game, a sword you acquired in the future is sent back to the past. The character who takes it promises to "seal it away", the concept of "ontological paradox" is apparently entirely foreign to the protagonists.
In Persona 3, it does have quite a devastating effect - the main female love interest wants to go back in time and save/see the main character before he sacrificed himself, which might bring about The End of the World as We Know It.
The main villains of Tales of the World: Narikiri Dungeon 3 are the main characters from the future, and they are messing up the timeline to unseal the "Demon King Jababa" and defeat him before he can destroy their village. The thing is, the game implies that he got out and destroyed the village because they released him.
World of Warcraft has a quest that plays with this. While doing a survey for the Bronze Dragonflight (keepers of time), the player is assisted by his future self. Later on, he needs to do the same thing again to protect his past self. There are also numerous instances which involve traveling through time to either prevent the Infinite Dragonflight (the apparently corrupted future version of the Bronze Dragonflight) from changing the past (the goal of which may finally be becoming clear in the latest expansion, to cause the world to come to an end to prevent it coming to a far worse end later on) or to retrieve things that were lost since great prior events involved them (and, it appears, were lost precisely because you went to get them).
Time Shift stays light on the plot side, but that doesn't stop the nastier issues from occurring. By the end of the game, the protagonist has been sent back in time and had his time machine damaged by a bomb that later never went off, sent back in time and had his or her time machine damaged by a giant mechanical spider that later was distracted somewhere else at the time and then ceased to exist, can accidentally lead to the death of an individual who earlier would go on to assist the protagonist, was assisted by and provided assistance to capture La Résistance who later never were captured and then were La Résistance to an entirely different matter. All to prevent a time traveling Mad Scientist from replacing 1940s Europe with his own special dictatorship which also prevented the time traveling Mad Scientist and protagonist's time machines from ever being developed. That's before we get to the Wild Mass Guessing that the protagonist survived the bombing once sans timesuit and then went back in time to try to stop the Mad Scientist from ever going back in the first place, or that the protagonist is a time-clone of one of the other characters as implied by the paradox warning triggered by attempting to remove his or her time suit. OWWW!
It does strongly note that the Beta Suit cannot ever benefit from the Reverse Time effects, nor can you benefit other people. For example, you cannot use Reverse Time to repeatedly kill/revive someone, your ammo/grenades will never return to you, your health won't restore from bullets never hitting you, and in one case, you cannot prevent someone from being pulverized by stepping into the path of a wind tunnel. You can, however, do some suitably amazing and freaky shit, such as reversing time and shooting from two separate locations simultaneously; once in real time, then reverse time, move to the new location, and open fire once real time kicks in again. The Beta Suit's "mission", however, is to recover the Origin Drive from Krone's Alpha Suit.
The InfocomInteractive Fiction game Trinity combines this with a Stable Time Loop: Moments before London is destroyed by a nuclear attack, you go back in time to sabotage the original Trinity test and prevent atomic weapons from ever being used.... Unfortunately, as your Spirit Advisor explains, that would change history so that you were never born, thus creating a paradox. The universe, fortunately, resolves the paradox by making a small explosion every time one of the atomic weapons that should not exist is detonated (i.e., destroying a city instead of destroying most of New Mexico as it should) and sends you back to London before the bomb is dropped to do it all over again....
In FAMOUS has a character who still exists despite drastically changing his own past. The idea of alternate universes is never so much as mentioned; rather, the character in question travels through time via his own superpower, with immunity from changes in the timeline granted by Required Secondary Powers. Unfortunately, the timeline itself is not immune to his presence, as the sequel will show, creating another Alternate Timeline...
In Prince of Persia: The Sands of Time, the Prince's father find the Hourglass of Time whilst invading another land's fortress, and the Prince is tricked into releasing the sand within by the evil vizier. This turns pretty everyone except the Prince, the vizier and the princess from this other land (Farah) into sand monsters, leading the Prince on a quest to undo it all. He teams up with Farah, but she dies during the adventure until he manages to get to the hourglass and insert the dagger, reversing everything up to the point where they originally invaded the fortress. Waking up in camp with the dagger he visits Farah and tells he everything, though she doesn't remember now naturally, since it never happened. The vizier enters, but the Prince kills him, then gives the dagger to Farah and leaves, asking her to call him "Kakolookiyam", a word with significance to her that she told him during the adventure together.
Cue Prince of Persia: Warrior Within, 7 years later. The Prince is informed by some old dude that he should have died during the first game, but cocked it up through messing round with the time continuum and is now hunted by the Dahaka, a guardian of the sands, which seeks to kill him and restore the balance. To stop this, the Prince decides to travel to the Island of Time to stop the sands ever being created by travelling back through time to kill the Empress of Time. He rescues a woman, Kaileena, and sets about trying to get to the Empress whilst dodging the Dahaka and a strange wraith-like figure seemingly out to get him. Whilst travelling to the Throne room he is confronted by both, but escapes when the Dahaka kills the wraith and buggers off. Kaileena reveals herself to be the Empress; the Prince kills her and travels back to the present, only to be confronted by the Dahaka again, since the sands turned out to have been created by the act of killing the Empress. He almost gives up hope when he discovers the mask of the Sand Wraith, which allows him to co-exist with himself in the same time-line. He then goes back in time, revealing himself to be the strange wraith-like figure, who wasn't trying to kill the prince but in fact save him. When confronting his past self with the Dahaka outside the throne room he dodges the Dahaka, allowing it to kill his old self, which reverts him back to the Prince. Confused yet? He then proceeds to confront the Empress again, but this time throw her through a portal into the present, planning to kill her here, thus still creating the sands, but not in a time frame that would allow them to be found by his father. Then the Dahaka shows up again, now trying to kill the Empress, but together they manage to defeat it and set sail for Babylon, the Prince's home.
Which takes us to Prince of Persia: The Two Thrones. Arriving at Babylon they find the place ransacked because by retconning his past the vizier of course never died, got hold of the dagger, and proceeded to attack it, looking for the Sands of Time. Kaileena is captured, but when the Prince tries to rescue her the vizier stabs himself with the dagger, turning into a sand god or something, killing Kaileena and infecting the Prince with the sands. Princey manages to swipe the dagger, though, escape, and set about to kill the vizier again. Along the way he bumps into Farah, who had been captured way back when the vizier got the dagger, and discovers that the sands have manifested within him as the Dark Prince: a separate personality that tries to convince him to look out only for himself. He catches up with the vizier, is soundly beaten and thrown into a well, finds his father, who is dead again, and has a crisis moment where the Dark Prince tries to take over. He resists, fights the vizier again, and kills him with the dagger; Kaileena appears and cleanses him of the sands, and all seems well. Then the Dark Prince pulls him into his own mind and tries to screw it all up but he resists, gets rid of him too and gets the girl. All's well that ends well. Aside from the dead father and ruined city.
The flash game Chronotron revolves around the players ability to travel back to the begining of the stage (so that multiple version of the player exists at the same time). It is quite possible to either kill a past self, or bar their passage to the time machine - resulting in a time paradox "death", complete with a penrose triangle warning sign.
According to Word of God, there's an active paradox known as the Split Timeline Theory in the Zelda universe. Basically, when Link defeats Ganon at the end of The Legend of Zelda: Ocarina of Time and Ganondorf gets sealed away by the sages, Zelda sends Link back into the past where he warns the king of Hyrule of Ganondorf's intentions and then leaves Hyrule. Now, this doesn't negate the need for time travel; instead that Bad Future remains, but without Link since he's back in the past. Much later in that future, Ganon escapes, and the events of Wind Waker happen, hence the "The people believed that the Hero of Time would again come to save them. / ...But the hero did not appear." in the prologue.
There's now an official timeline. There are three timelines: One if the Hero of Time lost, one if Ganon(dorf) was defeated after he took over, and one if the Hero of Time was sent back and he took care of everything before Ganondorf took over.
Millennia Altered Destinies has this as a premise. You are given a space/time ship called XTM that can travel to any star in the Echelon Galaxy within a 10,000 years timeframe in 100-year increments. Your ship is shielded from "temporal storms" which occur whenever you change something in the past. You also have a database containing information for each of the planets in the galaxy. Unlike the ship, the database is specifically unshielded, so that you can keep track of any changes you make and their consequences. The goal is to prevent the galaxy from being taken over by the Microids. In order to do that, you have to guide four sentient races from the Stone Age all the way to space travel. You also have an Evil Counterpart, which is you from an alternate timeline, who was recruited by the Microids to keep history from being changed. He/you will randomly appear at different points in time and mess up your efforts. And you can't kill him/you, as he/you will do a Hyperspeed Escape as soon as you show up. Occasionally, during a time jump, you can find yourself in a "green mist", facing an alternate version of you (not the evil one). Whatever you say to him now, he will tell you later, when you enter the "green mist" again. This is a clue to getting the most powerful weapon in the game. Originally, the developers included a Non-Standard Game Over (in the form of a temporal storm that's too powerful for the XTM) which would be kicked off if you messed up the game so bad it can't be fixed. However, they quickly realized that, the way the game is designed, there is no way to reach this situation.
In Day Of The Tentacle there are several examples. Thought temporal paradoxes are mostly averted during the main game, where the protagonists are scattered through three periods, some sever paradoxes occur after the second time-travel where the protagonists (and the tentacles) travel to yesterday.
The whole point of the plot is preventing an event from happening, thus erasing the need for the time-travel. However it is an essential part of the gameplay that timetravels do affect the present.
During the endgame (that is set one day before the present) it is possible to remove the "Help Wanted" sign from the window, that will be picked up from Bernard in the present. Also the bowling-ball should be in a different place.
According to the dialogue between the protagonists and Purple Tentacle, the Sludge-O-Matic, what made purple tentacle an insane genius, was invented by himself, who send it back to the present to Doctor Fred. (This makes their heads hurt)
Purple Tentacle brings a lot of future versions of himself back to the day before the present. So if one of them get hurt or can't return to the future, this would create some horrible paradoxes.
The entire plot of Dark Cloud is this. At the end, you have to go back 400 years in the past to erase the origin point of the Dark Genie because he's too strong to beat in the present. Of course, in doing so, you make sure that Seda's wife comes Back from the Dead. This prevents Seda from becoming overcome with fury and dark powerand getting possessed by the Dark Genie, removing his motivation to fix it all with time travel. Thus, he never rips a hole in time to get to the present and tell Toan about it. Which means that Toan's village never gets blown up, and hence, Toan never goes on a quest to destroy the Dark Genie. Which means that Seda gets possessed and goes into the present. Which means that he doesn't. Which means that he does. Which means that he doesn't. Which means that he does...
Result: There are an infinite number of Sedas who both do and don't Time Travel.
Keeping in mind that the last scene shows Toan returning home, it makes you wonder just what he's coming back to.
And before you even get to that, there's that whole ordeal in Queens. After the boss battle, Rando breaks the Life Sphere, intending to return 100 year to the past with La Saia so they can get married. Since when is Time Travel defined as a feature of the Life Sphere? We're left with the same problem: if Rando and La Saia get married, La Saia never commits suicide because Rando never got the Life Sphere, resulting in La Saia's ship never being sunk, which should completely remove the Shipwreck dungeon from the game, which would mean that the Turtle was never built, meaning that Toan never did anything in Queens. And without a dungeon to go to, there's no place for the Atla to end up, so the town itself was never destroyed.
It's possible Rando was only speaking figuratively; he's saying he wants to do what he should have done that day: marry her and screw what the world thinks.
The actual objective of Servant Archer/Heroic Spirit Emiya in Fate/stay night. He tries to kill his younger self (the protagonist, Emiya Shirou) to force a contradiction within Gaia, which he hopes will cause his whole existence to be erased to keep reality from breaking from the impossibility of the event. He himself admits that this would have a very low chance of happening, considering that by meeting Archer, Shirou is already set on not becoming Heroic Spirit Emiya, so the death of a "different" Emiya Shirou shouldn't affect Heroic Spirit Emiya in the slightest.
Besides, it's said that the Heroic Spirits are removed from the time axis and await their summonings in the Seat of Heroic Spirits. So, even in the case that Shirou actually did want to become a Heroic Spirit, as Archer is no longer bound by the rules of time, Shirou's death would not form a paradox and free him from his destiny. Archer's whole objective in UBW was both to attempt this plan anyway in the off-chance that it actually succeeded, and to make sure his past self didn't have to see his ideals betray him like he did.
Paradox is notably averted in Schlock Mercenary, where it's put forth that causality is never sacrificed. If a time traveler were to go back and kill his/her grandfather, even though another one of them wouldn't be born as a result, they've already caused the event, so they'll still exist. Changes due to time travel are likened to "overwriting" the original timeline (though it's also worth mentioning that, within the Schlockiverse, time travel is normally impossible).
The comic itself likens it to the most common time travel power in video games: "Load Game".
Evil Katarakis of Starslip Crisis hasn't thought his brilliant plan all the way through:
Imagine... enjoying your favourite sandwich... then going back and stealing the sandwich from yourself before you eat it! So you get two sandwiches.
Deep Time isn't exactly the best off, either: one member spent spare weekends going back in time to kill Hitler, then back again to stop himself from killing Hitler. Another spends forty years on dead-end research, then tells a third agent to stop him from wasting his life on the matter. In yet another case, accidentally blowing up a planet causes an entire Deep Time spaceship to spontaneously fall apart.
At one point, people in the present protect themselves against attackers from the future by identifying the future guys' ancestors, and making sure they're on all the ships that the future guys are attacking.
Red Mage accuses Chaos of having an unworkable plan because of a variant on this. Chaos intends to destroy all of space and time which would destroy all the events leading up to his being summoned to destroy all of space and time.
Reaction: I'd better not create a temporal paradox! I'd hate it if everything were destroyed the way I wanted it to be destroyed!
In Homestuck, most of the technology that can teleport an object thorough time and space are designed to prevent these paradoxes; appearifying an object that would cause a paradox instead creates a copy made of paradox ghost slime, which quickly collapses in a puddle of goo. It is possible to cause a grandfather paradox, but this creates a doomed Alternate Timeline marked for deletion - even if someone travels back to the main timeline somehow, they're still doomed to die.
Except for Davesprite, who seems to have dodged Fate's bullet by merging with a pre-existing entity in the main timeline, i.e. the kernelsprite.
Faulty Logic: Fox built a time machine for the sole purpose of traveling back in time to prevent himself from building a time machine, because he knew he would build a time machine and misuse it.
The Danny PhantommovieThe Ultimate Enemy is one big temporal paradox. In the original timeline, Danny's family and friends are killed, he goes mad with grief and kills himself (people with a Split Personality can do that and survive), and his evil self terrorizes the world for ten years. Thanks to Danny and some timely interference by the Dungeon Master, this timeline was erased, but his evil self was in the past when it was erased, so he still exists even with the events that caused his existence never happening. His evil self even pointed out the paradox. "You don't get it, do you? I'm still here. I still exist. That means you still turn into me." The Observants mention something about him still being there because "he exists outside of time."
In Invader Zim, an entire episode (Bad Bad Rubber Piggy) has one scene that demonstrates this perfectly: After GIR finds out that Zim intends to send a robot back to the past to destroy Dib, it leads to this classic line of dialogue:
GIR: Wait... if you destroy Dib in the past, then he won't ever be your enemy, so you won't have to send a robot back, so then he will be your enemy, so then you WILL have to send a robot BACK... (head explodes)
Super Friends. In the Challenge episode, "Secret Origins Of The Super Friends," the Legion of Doom tries to change history by messing with the origins of Superman, Wonder Woman, and Green Lantern. Okay, but seeing as how much of Super Friends is based on Pre-Crisis on Infinite Earths lore and hence Luthor's baldness and Start of Darkness were both accidently caused by Superman when he was Superboy, how can Luthor— and as its founder, the Legion of Doom itself—exist if Superboy was never there to cause what happened to Luthor? Likewise, given his origins even Pre-Crisis involved someone copying Superman, how does Bizarro continue to exist as well? This also applies, to a lesser extent, to the others. Sinestro's crimes were exposed by Hal Jordan; maybe eventually, someone else would have, but it was Hal's newbie attitude that caused him to question an otherwise model Green Lantern; shift the timeframe and he maybe never meets the LoD. Most Cheetah origins have Wonder Woman involved in some way, at least some as Cheetah feeling challenged by her existence. Also, take Luthor grabbing Abin Sur's ring. That might get the Guardians' attention. Also, without those three, and especially Big S, would the SF have even formed, and since the LoD formed out of fear of this group…the list goes crazy on. Ah, everyone know time travelers are surrounded by a temporal bubble that prevents them from being affected by their own alterations in the timesteam. The real question is: if the Legion of Doom could see through time to spy on the "secret origins" of the heroes, how do they not know the entire Justice League's secret identities?
Ontological paradox examples
Anime and Manga
In The Disappearance of Haruhi Suzumiya, while traveling in the past Kyon is stabbed by Ryoko Asakura. As he lies bleeding out on the ground, what appears to be a Kyon from the future comes with a Yuki and a Mikuru also from the future and rescues him. So, basically, Kyon only lived because he lived long enough to go and come back to save himself. He lived because he lived. My head hurts...
Don't forget the information paradox with the knowledge of Mikuru's mole. Kyon didn't know Mikuru had the star-shaped mole until future!Mikuru showed it to him. Mikuru herself didn't know until Kyon told her about it. When future!Mikuru realizes this, she is understandably upset, thinking she messed something up.
Or the "Endless Eight" story arc, which finds the central characters reliving the same eight day cycle 15,498 times (quite unbeknownst to anyone but Yuki). They finally break the cycle when Kyon suggests a suitable ending to their summer vacation to Haruhi.
Transformers Armada, in the "Drift" episode. Starscream is blasted with the Requiem Blaster, then Highwire somehow apparently warps the kids back in time, but in an Alternate Universe, where both the Autobots and Decepticons are imprisoned and slowly being digested within Unicron. Before he expires, Hot Shot reveals that the Minicons are actually Unicron's cells, and the Transformers were being used by them. Then the kids travel further back in time to when the Minicons were created. Here they tell them to escape from Cybertron, eventually resulting in them coming to Earth and all subsequent events in the story. Then, back in the present, Perceptor stops Thrust from blasting Starscream. Therefore, the kids had to go back in time to trigger the sequence of events that led them to Cybertron and ultimately the time travel event itself.
In The Bardock TV Special Bardock attempts to stop Freeza from destroying Planet Vegeta to prevent the creation of a Super Saiyan. He fails. In the Episode of Bardock spinoff it turns out that Bardock wasn't killed in the explosion but was sent back in time to before the Saiyans discovered Planet Plant. He fights Chilled, Freeza's ancestor, and during the fight he becomes a Super Saiyan. This means that Bardock is the Super Saiyan of legend, and that Chilled was the one who passed the legend down to King Cold and Freeza. That in turn means that Freeza destroyed Planet Vegeta because Bardock became a Super Saiyan when he fought Chilled.
Tsubasa Reservoir Chronicle has several such paradoxes. One of the most noticeable is Real!Syaoran, who is a living, breathing time paradox, what with being he's the son of his own clone. It's implied that the timestream was desperately trying to hold itself together, resulting in a few Stable Time Loops to patch up other paradoxes, resulting in Real!Syaoran's existence. When everything is sorted out in the end and the multiverse is repaired, Clone!Syaoran and Clone!Sakura are RetGonned from existence, and Real!Syaoran almost ceases to exist because if his clone never existed, then neither could he, and he only ever really existed in the first place because he went and broke causality. The same thing goes for Watanuki, who only started existing to fill out a hole that Syaoran went and made in the multiverse by time-traveling. Now they've both got to pay for the repairs, so Real!Syaoran exchanges his ability to stay in one world for very long for his right to continue existing, while Watanuki instead trades his ability to go anywhere ever except Yuuko's shop.
Booster Gold only becomes Booster Gold because as Michael Carter, a janitor in a 25th-century superhero museum, he steals a timesphere belonging to the time master Rip Hunter. It later transpires that Booster will father Rip Hunter and teach him everything he knows about time travel. So if he hadn't stolen the timesphere, the timesphere wouldn't have been there to steal in the first place. Augh. To complicate matters, Rip has to train Booster to be a time master so that Booster can have trained him to be one when he was a little boy.
In Pre-Crisis Superman comics, all time travel works this way, which is why Superman's ability to time travel by exceeding the speed of light is not a Game Breaker; he can travel back to the past, but he can't successfully change anything. The Movie ignores this.
In Back to the Future, Chuck Berry steals Johnny B. Goode from Marty (after hearing an incomplete performance over a 1950s payphone, no less), who learnt it from Berry in the first place.
Another Sandra Bullock film, Premonition, mixes this trope with You Can't Fight Fate: Linda's attempts to prevent her husband's death cause it, but she does get pregnant before he dies, and prevents herself from going crazy and getting committed, which she could not have done had she not had the premonitions of the future.
Referenced in Déjà Vu by agent Carlin right before they send a note back in time:
Technician: It would have gone faster if you had written it [the note] yourself
Carlin: Yeah, then I recognize my own hand writing and the universe explodes.
The Terminator: Fathering the guy who will send you back in time counts, too. Also, where did the message about how "the future is not set" originate? Chronologically, Kyle first gave it to Sarah in 1984 as a message Future-John had sent with him when he sent him back in time. Then Sarah decides to teach it to present-John when he is a child so that he will have the message to send. So she gives the message to John, who gives it to Kyle, who goes back in time and gives it to her, who gives it to John, who gives it to Kyle, who goes back in time and gives it to her, who oh look I've gone cross-eyed.
In Minority Report, Precogs can predict murders before they happen (hours for crimes of passion, days when premeditated). The protagonist finds himself being accused of a premeditated murder of a man he has never met. He naturally assumes himself to have been framed. To this end, he studies the visions of the Precogs to track down this individual. It is these acts which make the murder premeditated, since he's actively trying to find this guy and will supposedly end up killing him. This leads to the Fridge Logic problem with the entire movie: how the hell could Burgess have arranged such a thing? The last time he used the Precogs to do such a thing, he did it in a wholly logical way. His framing of Anderton is wholly illogical, since there's no apparent source for the vision and no way it could have happened naturally. The setup is entirely dependent on events that never would have come to pass without Anderton having been present to see the precogs' vision; their prediction itself is the very cause of what they predicted.
Timecrimes could have been renamed Ontological Paradox: The Movie. The protagonist travels backwards in time, and ultimately ends up responsible for the events that caused him to travel back in time.
In The Time Machine (2002), the uber-Morlock explains, "You built your time machine because of Emma's death. If she had lived it would never have existed, so how could you use your time machine to go back and save her?"
Summer Time Machine Blues is a Japanese film that starts with a group of high school students on a hot summer day stumbling upon a time machine and using it to prevent the remote control for their air conditioner from fizzling out due to a spilled coke can. Hilarity Ensues.
An Object Loop gets lampshaded when Kirk pawns his reading glasses in 20th Century San Francisco.
Spock: Excuse me, Admiral. But weren't those a birthday gift from Dr. McCoy?
Kirk: And they will be again. That's the beauty of it.
There's also an Information Loop, in the form of Scotty providing the formula for "transparent aluminum" to a 20th century scientist, who, it is implied, will go on to "invent" it. The novelization explicitly states that the scientist they give the secret to IS the historical inventor of transparent aluminum, which was only the beginning of his accomplishments, and Scotty observes that it might be ESSENTIAL that they give it to him. In the film it's merely hinted at: McCoy complains about giving the scientist the formula and Scotty replies "How do we know he wasnae the one who invented it?"
Willow contains a predestination paradox (if you assume, as the film does, that prophecy really is knowledge of the future): Bavmorda's attempts to destroy Elora are the very thing that causes her own destruction, which she would not have attempted to do EXCEPT for foreknowledge that Elora was going to cause her destruction.
Live Action TV
As LOST season 5 deals with a Stable Time Loop, this type of paradox is emerging. Kate, Sawyer, and Juliet save Ben's life, allowing Ben to grow up and turn the wheel, which causes the time travel in the first place. There may be physical examples as well: in the future, Richard gives Locke a compass. Then Locke travels to 1954 and gives it back to Richard. While it's possible Richard now has two compasses (and must later give Locke the "newer" one,) or the compass was never created.
The other major season 5 storyline has a similar problem. Jack's goal is to set off a bomb that will prevent their plane from crashing, meaning they'll never come to the island; completely erasing everything that's happened on the show. This means Jack will never have been there to detonate it. Interestingly, it is suggested that the explosion may end up doing the opposite of what Jack wants and leads to the plane crashing. The blast ends up creating an alternate timeline where they never went to the island.
Since the flash-sideways were revealed to be purgatory, it is more likely that the blast was responsible for (or at least contributed to) the "incident" that ultimately led to the plane crashing on the island.
Happens in "Catch-22" when Desmond set out into the jungle after a parachutist on the island and made sure all of the details of his quest exactly matched his vision of the event. The events of the vision only happened because he saw the vision. Also, Daniel Faraday is sent by his mother, Eloise Hawking, to the island, even though she knew full well that she would be the one to kill him when he arrives.
The Compass is a textbook example of the abovementioned Object Paradox: looping endlessly between 1954 and 2007.
In an episode of Wizards of Waverly Place, Harper travels back in time from the future. At the end of the episode, present Harper sees a hat that future Harper is wearing. She asks where she got it, and future Harper gives present Harper the hat. But then you begin to wonder where the hat came from in the first place.
As of the season 3 finale of Fringe it is revealed that the machine left by the first people actually came to be scattered in the past by a future Walter Bishop who sent his machine back through a wormhole leading to the paleolithic era. This means that the machine both came from nowhere and is infinitely old.
Another Doctor Who example: In The Big Bang the Doctor avoids the physical paradox by throwing away a note written by his future self and placed in the past, but the information on the note is still the source of the information on the note.
Then, there's the comic relief special Time Crash: the Tenth Doctor and the Fifth Doctor meet. Ten knows what to do about a certain problem because when he was Five, he had experienced these events and watched his future self do it. When one of the Doctors points out how that makes no effing sense, they both say "Wibbly-wobbly, timey-wimey" and move on.
Even more gloriously: in the Trope Namerepisode for Timey-Wimey Ball, The Doctor was just reading from a script in the past, written down by the protagonists in the present and given to him in the future, so that both sides can communicate across 40 years (he leaves a recorded message, with pauses for the protagonists to "answer," they write down what everybody is saying, they then give the transcript to the Doctor in the future so that he knows what to say on the message and what they're saying back). In this, he famously describes time as a "ball of wibbly-wobbly... timey-wimey... stuff," allegedly pausing because he can't think of what to say, but actually embarrassed that the Stable Time Loop invented THAT to require him to say under penalty of breaking the loop. He did not describe time that way, TIME described time that way!
In Red Dwarf, Kryten's last words (in a timeline that is eventually undone) are the words he knew he would say because he saw his future self die earlier in the episode. Somehow, though he only said it because he knew he was going to, "enig" turns out to be important, short for "enigma." Of course, Red Dwarf is hardly anyone's idea of "hard SF."
A Reverse Grandfather paradox occurs in the Farscape episode Kansas: In order to convince his dad to not go on the Challenger mission, Crichton sets up a scenario he remembers from his childhood where he was trapped in a burning building and his dad saved him. When the time for the rescue actually occurs, Dad gets injured and older John has to save both his father and his younger self.
First is the Fourze Connect Ring. When Future-Gentaro comes upon the Wizard plot, he simply pulls the ring out of nowhere and uses it to help Haruto fight off the film's Big Bads. Before returning to his own time, Future-Gentaro asks Haruto to give his present self the ring (as well as the present Fourze Driver, which he borrowed from his present self).
Second, and more confusing, is Gentaro's class photograph. Future-Gentaro says that he was inspired to become a teacher by a picture of himself with a class of students; when he shows it to his friend Kengo, the latter remarks on how dog-eared it is. When Future-Gentaro pulls his past self asideto borrow the Fourze Driver, the photo falls out of his jacket, and at the end of the movie we see that Present-Gentaro found it. While we do see the photograph being taken, this happens after Future-Gentaro returns to his time, meaning the photo still technically appears ex nihilo.
Chronos, the Incarnation of Time from Piers Anthony's Incarnations of Immortality is immune to this, to an extent. He cannot be balked by paradox, he remembers the original and the new timeline, though no one else does. The limit is that he cannot interfere with his own workings (the "Three Person Limit"). He can exist once, go back in time and change things, but he cannot go back in time and stop himself from changing things, thus the three person limit.
The ultimate time paradox story is Heinlein's—All You Zombies—, in which the protagonist turns out to be hisheritthey's own mother, father, son, daughter, grandmother, grandfather, grandson, granddaughter, great-grandmother, great-grandfather, great-grandson, great-granddaughter, great-great-grandmother, great-great-grandfather, and so on, ad infinitum. Also hisheritthey's own recruiting officer to the Temporal Bureau.
Another Heinlein story, By His Bootstraps, takes things nearly as far. Among other hijinks, the main character gets a book from the future, which he copies into another one (the same one, when it's new?) when it becomes too old and falling apart. A good way to avoid an object-based ontological paradox.
Averted — by the characters, no less — in Isaac Asimov's short story The Red Queen's Race. They wind up creating a Stable Time Loop instead. A scientist conducts an experiment to send modern scientific texts back in time, translated into ancient Greek. His translator, fearing a Temporal Paradox, only translates the parts that would account for the oddly anachronistic scientific advances already in our ancient history, like Hero's steam engine or the infamous Baghdad Battery.
In Artemis Fowl and the Time Paradox, Opal Koboi from the past travels to the present, and possesses Artemis' mother, making her appear ill. This forces present day Artemis to travel back in time to get the cure from the past Artemis. Opal then uses Artemis returning to the present to return to a few days before the present to make Artemis' mother ill in the first place. Ironically, this is all so she can aquire the secret of time travel.
Not to mention, Artemis had foggy memories of the past. When he went back in time, he left a note for Mulch to open the trunk Artemis and Holly were locked in. Also, the Mulch and Artemis of the past had their minds wiped, and since Artemis' wipe was a blanket wipe, there were still several remaining facts about fairies. By travelling back in time, Artemis caused himself to discover the fairy race. Whoa.
Played with in the latest Thursday Next book, where they find that despite the existence of the Chronoguard, no one has actually invented time travel yet, so they assume that the technology much have been sent from the future and eventually they'll find the spot on the timeline where someone invented it to close the gap. As one character describes it, it's like they're running the technology "off of borrowed credit." This causes trouble however, when the Chronoguard begins to realize that no one in the timeline ever invented time travel. The resulting paradox causes the system to unravel and gets rid of any further possibility of Time Travel in the series (although it seems everyone in the populace has a Ripple-Effect-Proof Memory).
In Harry Potter and the Prisoner of Azkaban, Harry and Hermione travel back in time for a number of reasons. During this time travel, Harry manages to save himself from dementors using an Expecto Patronum charm. The event is noted to have happened earlier in the book with Harry only glimpsing his mysterious saviour and thinking it looked a lot like his dad.
In Flatterland (a Spin-Offspring sequal to Flatland), Victoria Line and the Space Hopper end up trapped in a black hole. They're rescued by slightly older versions of themselves with a portable white hole, producing both a reverse grandfather paradox and an object loop.
There's a human version of the object loop in Pyramids, with Dios (who frequently makes reference to a lack of memory very far back) being transported backwards through time to the beginning of Djelibeybi. There's also some Reverse-Grandfather involved, considering he persuaded the original founder of Djelibeybi to begin the Pyramid tradition, which in turn allowed Dios to live long enough to go back in time to persuade the founder and so on...
In Harry Harrison's The Stainless Steel Rat Saves the World, Jim comes home to find that sending He (that's the villain's name) to the time when Earth was about to be destroyed led him to launch the Time War, yet the Time War is the reason Jim got involved in the first place.
Jim: The way I see it, He just bounces in a circle in time forever. Running from me, chasing me, running from me. . . . Arrrgh! When was he born? Where does he come from?
Coypu: Those terms are meaningless in this sort of temporal relationship. He exists only within this time loop. If you wish to say it, though it is most imprecise, it would be fair to state that he was never born. The situation exists apart from time as we normally know it.
Unsuccesfully invoked and thereby subverted in Calvin and Hobbes: Calvin tries to travel two hours into the future so that he won't have to write the story they're supposed to be writing for school. But the future Calvin doesn't have it, because he was to busy time travelling to the future to actually write it. Then they both travel to one hour ago because they decide that that Calvin should have written it... but he refuses on the grounds that whatever they threaten to do to him, they'll be doing it to themselves. In the end, the two Calvins return to the future empty-handed, only two find that the two Hobbeses have written the story for them. When Calvin starts reading it out loud at school, it turns out to be a story about his foolish time-travel while the tiger(s) save(s) the day.
The timeline of this whole thing is a little paradoxical, but at least the object/information obtained has an origin.
In Achron, pulling one of these off yourself isn't as complicated as it sounds. Usual example: You build a chronoporter (a time teleporter) at a point in time. However, your opponent, who is further in the past than you, destroys your chronoporter. Since chronoporters are expensive and essential to most late-game tactics, you send some units back in time to defend the chronoporter. Instant ontological paradox! The chronoporter only survived because it was defended by units it would chronoport later on. Bear in mind that time travel is fully logical in Achron, so what seems like a paradox makes perfect sense in-game.
In The Legend of Zelda: Ocarina of Time Link learns the Song of Storms from a man who claims that he learned it when a kid played that song seven years ago and messed up his windmill. Link then travels back in time seven years and plays the song, messing up the man's windmill. To even further compound this, at least from the viewer's perspective, the background music within the windmill is the Song of Storms... even before Link learns the song.
The Legend Of Zelda Oracle of Ages features a few instances, notably during one of Link's interactions with the Gorons: In an item trading puzzle, you trade a rock briquette to a goron in exchange for a family heirloom and then trade the same heirloom to an ancestor of the previously mentioned goron so he can hand it down across generations to his descendant who trades it to you so you can trade it to his ancestor who hands it down to him who trades it to you...
Shin Megami Tensei: Strange Journey has a rather curious form of this. In the second sector, you find a group of Disir whose time-control powers have been stolen by Yggdrasil. They send you to the future to battle him, but since he's so powerful, he'll just utterly curb stomp you with a hearty Evil Laugh. Moments later, as you drift into unconsciousness, another guy pops up in the battlefield and starts hitting Yggdrasil. Moments later, you return to Sector Two, and the Disir tell you the battle's now engraved in your destiny. Fast-forward to Sector Five, and you find yourself in a very familiar battlefield, with a sleeping Yggdrasil, and a destiny goddess who reminds you you have to save yourself before Yggdrasil kills you in the past...
Shin Megami Tensei IV has you interact with the past to a degree in the DLC missions. By finishing them, you prevent the Archangels from exterminating Past Tokyo and allow Mastema to imprison them in Kagome Tower, from which you spring them in the normal game. Most notably, you have to travel in time just a few hours or minutes after your past incarnation died attempting to convince Masakado to shield Tokyo from the ICBM cluster and defeat him to snap him to his senses, allowing literally all other events in the timeline to unfold naturally.
The issue of paradox is averted in Second Sight. Throughout the game, it appears that John Vattic is coming across information pertaining to peoples deaths, and then projecting himself back through time to avert them. The finale reveals that the parts which Vattic thought were the "present" were actually potential futures he was seeing through precognition. So he was predicting deaths which hadn't actually happened, rather than averting deaths through time travel (which would create the paradox of why he would need to travel back in the first place.
When you first meet Mender Lazarus in the City of Heroes Ouroboros initiation arc, he tells you during a mission fighting Shivans, you somehow knew to destroy the meteor. That mission is the last mission of the arc, and destroying the meteor is one of the mission objectives. Which you know to do because he told you. (Incidentally, that mission is the first time he met you.)
Max's "future vision" ability, in Sam & Max Season Three, allows him to pull puzzle solutions out of thin air.
Dragon Quest V makes use of the Reverse Grandfather Paradox: you travel back time to meet your child self, who keeps a piece of Applied Phlebotinum that would prevent the world from total annihilation. You switch that piece with a fake, knowing that the Big Bad will kidnap your child self and destroy the thingie.
An example of the Object Loop occurs while recovering the key to Karazahn in World of Warcraft. The key, as found in the present, was beyond the skills of Khadgar to repair and so was taken back in time to be given to Medivh. Due to the damage suffered by the key, Medivh could not immediately repair it, instead giving the player a spare key. The key he was repairing would be given to Khadgar, to continue its trek into the future to be broken and taken back, ad nauseum.
What happened to the blue Chaos Emerald? It starts the game in Elise's possession, but she gives it to Sonic as Eggman abducts her, later receiving the emerald as ransom. Later, Silver finds it in Eggman's base and uses it to travel back in time with Shadow, but the last thing they do with it is give it to Elise... Completely pointlessly, too, since they had just found the gray emerald in the past, so they could have left that one with her instead.
Mephiles the Dark decides upon his name when Shadow addresses him by it. Mephiles then goes back 10 years and introduces himself to Shadow, who learns his name. Thus, Mephiles learns his name from Shadow and vice versa.
The Journeyman Project series works on information paradoxes. The Time Police protagonist is only prompted to go back in time when monitoring devices report historical alterations. Thus, once his mission is completed, there was never a reason to go back in the first place. The only major loophole the series provides is the rule that anything travelling back in time while a temporal overwrite is moving forward (We know, we know) is rendered immune to causality.
Ubiquitous in Infinity series:
Ever17: Information Loop in Blickwinkel telling You'haru the plan to save Takeshi and Koko, and You'haru explaining him the plan 17 years later.
Remember11: Both Information Loop and Object Loop with Yuni's terabyte disk, which travels endlessly between 2011 and 2012 and back. Possibly Reverse Grandfather Paradox in "player's" involvement in the story, as "it" corrupts Sayaka, causing Satoru to devise a plan to summon and send it back in time, making "it" corrupt Sayaka...
At the climax of the Doc Gets Rad chapter of The Adventures of Dr. McNinja, villain Sparklelord is sent back in time to the moment when he originally entered from Another Dimension. This version apparently overwrites the original copy of him, but without any accumulated memories, thus condemning him to repeat the same sequence of events for eternity. So there is an infinite quantity of him entering the loop, but nothing coming out... huh? Presumably, the only reason the universe doesn't implode is that the comic runs on the Rule of Cool.
Trying to understand a complicated series of events in Irregular Webcomic! seems to lead to this conclusion. Two characters are captured for Organ Theft purposes. Their future selves come to save them, but end up being captured as well. The original pair having their organs stolen survive by stealing the organs from their future selves, but eventually come across their original organs, and put those in them as well so that when their organs are stolen, only the spares are taken. It's all very complicated. Of course the characters end playing a part in destroying the universe by destroying the only time machine in existence instead of using it to become their future selves.
In thisFaulty Logic comic the main character builds a time machine to steal ideas for comics from his future self, only to have them stolen by his past self several seconds later. In essence, these ideas only exist for several seconds, over and over again.
Homestuck contains probably the mother of all ontological paradoxes: The Heroes create their parents from their own genetic material, and then create themselves from their parents material, then send the parents and children back in time to become themselves. There's even a term for this: Paradox Cloning, when a person is cloned from themself.
It should be noted that those examples happen all the time. Locally to John's session a group of previous players make contact because one of the players, Terezi, found money that was sent to her on her orders. They only look for this money because later John and his group will send an omnipotent demon into the previous playing session. This previous group of players, once aware of the humans, use internet messaging to talk to them all over the course of their lives, usually in heavily non-linear fashions that create stable time loops either way.
Worth noting that Terezi told Dave to wire her past self money so that she would discover him and be able to tell him to wire her the money.
For a more minor example, at one point Jade complains to John that her pumpkins keep disappearing so John sends her some pumpkin seeds for her birthday. However, these get sent back in time, and it is receiving that present that inspires Jade to start gardening in the first place.
For another, we have one of John's conversations with Karkat, who is trolling him backwards through time. Karkat claims that John told him that humans hatch as slugs instead of being born, and John tells him that's completely false but thanks him for the pranking idea. Sure enough, in John's next conversation with Karkat, he tells him exactly that and Karkat believes him.
For another, much later on Karkat opens a memo only to be distracted by himself from ten minutes into the future angrily responding to it. Throughout the course of this conversation present Karkat becomes characteristically enraged and when it's over goes to take it out on himself from ten minutes ago, starting it all over again. Future Karkat even lampshades this, stating that the whole bad mood basically sprung from nowhere and wondering whether it's even real. Yeah, Homestuck likes to play around with time a lot.
There's even a whole class of artifact whose defining feature is being an ontological paradox. A juju is a supernatural artifact whose origin is impossible to trace specifically because it exists in a Stable Time Loop. One in-universe theory supposes that they are spontaneously created ex nihilio by Paradox Space. Lil' Cal is a juju, one whose Stable Time Loop encompasses at least three separate universes.
S.S.D.D, Doctor Cook claims that he got on the Maytec board of directors using stock market information from a PDA that was accidentally sent back in time. But then he locked up the present day version of the PDA and made sure it was never sent back, he noted that the future version didn't disappear or anything.
Also the Anarchists were prevented from stealing the Wildfire time machine and using it to build the Inglourious fifty years earlier. Unfortunately they still have it, centuries before it wasn't built.
From Bob and George, this ontological paradox shows the time-traveling X and Bass giving Dr. Light and Dr. Wily the ideas to create them in the first place.
In Educomix, two objects - Jessica's mask and Dave's fez - literally come from nowhere, having been given to their original owners by people from the future. This causes a "Time Fart", which drains energy from other universes to keep the paradox in place.
In Wicked Powered, much like the "All You Zombies" example, the main character becomes both of his own parents through time travel, gender-bending, and amnesia. His DNA therefore has no origin, and he is immortal because of this.
In Jack, Drip unwittingly gets a chance at one, but he blows it and closes a Stable Time Loop instead. The Devil offers to trade Drip the scarf his mother wore in life in exchange for the murder of a married couple. Drip obliges and goes to kill their infant son, too, but the Devil tells him not to; instead, Drip decides to leave some disturbing imagery for the police by leaving the baby among the scattered carnage of what's left of the wife. It's not until Drip sets him down that he realizes that the people he brutally murdered were his mother and father, the crying baby looking up at him is his infant self. Drip is horrified to realize that not only did he actually kill his parents, he's also the reason he was sent to live with the grandmother who's been sexually abusing him all his life (and, having died and ended up in the area of Hell that Drip rules, is still doing it). The Devil points out that time works funny in Hell; Drip could have prevented it all if he had just chosen not to murder two strangers for a scarf.
Tom Francis' "Exploded" uses the "information" variant. Two guys invent a machine that predicts how one can die. While one can postpone one's predicted death, one cannot avert it entirely. The invention makes both men fantastically wealthy, and miserable.
The Reversed Grandfather Paradox is lampshaded for all it's worth in Red vs. Blue when Church is send back in time and attempts to prevent the accident that started the entire time travel problem.
Captain Flowers dying from a heart attack in his sleep? He died from the heart medicine Church gave him to prevent that.
The tank's AI named Sheila?
Tank:"Welcome to the 'M808V Main Battle Tank', you may call me 'Philis'."
Church:"What? Your name is Sheila!"
Tank:Name overwritten. You may now call me Sheila."
Sheila killing Church because of the friendly fire setting? Guess who changed the default setting.
In the first Futuramamovie, the "paradox-free time travel" isn't quite paradox-free: there remains an ontological paradox surrounding the origin of the name "Lars," as future-Fry chose that name when he realized that the injuries he sustained when Bender attempted to kill him made him Lars. From whence did the name come? No one knows.
I do. Fry was trying to say "Ow, my larynx", but the damage to his larynx made it sound like "Lars". The original Lars must have chosen the name at that moment. Paradox, schmarado- Oh, wait. The original Lars wouldn't have existed since without a Lars to send Fry into depression he wouldn't have gone back in ti- Oh, no. I've gone cross-eyed.
Well, if we just accept that there is no 'beginning' to the Lars and Fry cycle, then it all works out nicely. Fry says "Ow, my larynx", but it comes out as "Lars", at which point he realises "Oh hey, I look and sound like Lars now, I guess I am Lars!" Oh good, my eyes remained normal and my brain didn't explode.
Additionally, the temporal tattoo is an ontological paradox. At the end of the movie, Bender peels it off Lars's ass, takes it back in time, and puts it on the present Fry's ass. Thus, it was never actually created, and just loops through time eternally with no beginning or end.
Fry's genetic material. Being his own paternal grandfather, the question is where exactly the information carried on his y-chromosome originated.
Not only that, he went back in time using the same method as in the aforementioned movie. So much for "paradox-free" time travel.
This brings along Fridge Horror, since this means that in about 20 years, there will be an apocalypse led by robots.
Most if not all of the above is due to said "Paradox-Free" time travel actually being "Paradox-correcting" time travel: paradoxes come into existence repeatedly, but radiate enough doom that the universe actively kills them before they can do any damage. The only time this backfired on the universe was when the army of Benders was "corrected" all at the same time.
David Xanatosplanned out his fortune by time-travel. While in the past, he had a mundane coin and a note sent to his future self. The coin becomes a rare object worth a few thousand and the note contains instructions on how to invest the money.
The Archmage goes back in time, rescues his past self from falling into a chasm, then schools him on how to acquire the objects that gave him the power to, among other things, go back in time and rescue his past self from falling into a chasm, then school him—
The Phoenix Gate. It goes through all the events of the series until, to get it away from Puck, Goliath sends it to the distant past where it eventually gets discovered in the first place. The loop is looped.
This happens in the My Little Pony: Friendship Is Magic episode "It's About Time". Twilight Sparkle meets her future self, who tells her that she was able to get here because of the time spells located in the Star Swirl the Bearded wing of the Canterlot Archives. Later in the episode, she goes there (for entirely different reasons) and ends up using a time spell...to go back and tell herself about the time spells. Hmm, now where did she learn about the location in the first place?
The whole episode is this. Past Twilight is so bewildered and amazed by the concept of time travel that she can't shut up, and future Twilight doesn't manage to tell her what the actual thing she's supposed to be averting is before she gets sucked back to the future: all past Twilight knows is that future Twilight was from the following Tuesday. So she spends the week panicking about it, ends up with all the injuries future Twilight had when she visited, and by Monday night concludes the only way to stop whatever will happen by Tuesday from happening is to stop time. So she goes to the archives, but as Tuesday morning arrives, nothing happens, but she finds the time spell and goes back in time to warn herself not to worry about the future...
The Star Trek: The Animated Series "Yesteryear" revolves around a Reverse Grandfather Paradox in which Spock prevents his own death as a child. He doesn't do it quite right this time around, resulting in a slightly revised timeline when he gets home. Originally, his pet had lived. This time, he arrives a moment late, and the pet dies.
In The Transformers G1 episode War Dawn, the Aerialbots are sent back in time trying to destroy the Decepticons time machine. This put them back at the start of the war, right at the start of one of the very first Decepticon attacks. While they try to stay out of the way, the end up saving a insignificant, not important dock worker. As it turns out that insignificant dock worker was the un-upgraded Optimus Prime, both giving the Autobots a leader and keeping the Decepticons from winning an important first battle, getting an advantage over the Autobots and likely winning the war. It was Optimus who had the Aerialbots created in the first place.
Improperly invoked in Light and Dark The Adventures of Dark Yagami, after Blud learns that Matt survived a car crash with "Yotsuba", he decides to write Matt's name in his Death Note in the future to kill him in the past. This results in the past changing, with Matt dying and Yotsuba surviving. Dark claims the reason why Blud is telling him and Light this now, rather than at the point in the future when he writes the name is "Its one of those time parradoks that they have in Back to the Future".
Dark's exact words are "Oh I didn't tell you my death note can also kill people in the past and I am going to write his name in it in the future to kill him in the past and stop him stealing the death note." Hope that clears up any confusion. It doesn't help that the flashback scenes go from "Present Day" to "Meanwhile in the Past" to "Back in the Future"
In Megaman Star Force Orion, Amaya and Taisaka decide to go back in time to prevent Kiri from making contact with the Ice Goddess Talisman. With the help of the UMA Fire, they reach the year in which Kiri is exiled from her home. Fire then brings them 2 years forward in time, and Taisaka and Amaya meet Tagekai, who reveals that Taisaka was originally a member of Tri-Clan. This causes Taisaka to break down as Amaya abandons Taisaka, soon meeting Takeshi's former self. Takeshi later reveals he has memories of everything that occurred as he was 66, as he is trapped in that age. Taisaka changes herself by telling her younger self not to fight Tagekai which causes her to become exiled. Amaya and Taisaka bring Kiri to the Omnikron Temple, and Amaya meets Eidaya, explaining everything. This creates a major time paradox, which causes the already fragile sands of time to become even more fragile. Soon Amaya returns to the past, meeting his father, who realizes that Amaya is Amaya, and he travels after him. As Amaya tries to fix things in Tri-Clan, Takeshi reveals he killed Taisaka, and Daisuke King, a Time Traveling Kamen Rider, shows up and brings Amaya to the present. He warns Amaya that Ryo is about to be killed by a Shinigami named Albano, and if this were to happen, the future would be corrupt, as Takeshi would cease to exist in the present. At the same time, Amaya's father plans to force time into 11:60 PM on December 24th. Finally, Taisaka travels to the present from the past, creating a temporal corruption where memory demons overcome the present world and attempt to end the world. Time is eventually reset with the use of the Stolen Pocketwatch from the very first episode.
With the Tenth Doctor traveling with Holmes and Watson in Children of Time, temporal paradoxes are a concern, as the Victorian duo do have fates to fulfill in their own time. The season finale revolves around this, specifically, what happens when something that should not have happened does.
Millennium concludes with a massive paradox barrelling its destructive way into the future whose time travel efforts caused it.
Played with in Primer. As one of the characters says, "The last revision is apparently the one that counts." We find characters gradually losing their worries about causality; they wind up going back in time to relive the events of that same week in their original place — apparently intending to do everything right this time. It appears that causing a paradox causes some kind of mild brain trauma to the time traveler involved. But then there's that other version of yourself that you drugged up and locked in the basement so you could replace him...
Déjà Vu contradicted itself on terms of this. First, it is implied that anything changed in past changes the present, as Doug causes the death of his partner, that was thought caused by the ferry explosion. Later, it is implied that the past has already been changed, as the message "U CAN SAVE HER" in Claire's house was written by him, but in the end, it is contradicted, because if he prevented the explosion, he could never have been assigned to the case, and thus could never do the time travelling, and so on...
French-Canadian movie based from a cult tv show Dans une galaxie près de chez vous 2 featured a spatio-dimensional rip (shaped like a zipper) who goes to present Earth. The Capitain was able to chuck down a DVD with their plea (NOT to destroy the ozone layer) recorded on it. It backfired when the video got featured on YouTube and ridiculed as "Star Wars Twit" (Being bad at pronounciation dosen't help). Nevertheless, it might have pushed a younger version of the Capitain to go into space, directly and indirectly setting the events of the show into place.
Austin Powers: The Spy Who Shagged Me shows Austin briefly attempting to reason why no time paradox has occurred due to he and Dr. Evil time traveling to a date where they logically shouldn't be. Basil Exposition puts his mind at ease
Basil: "I advise that you not worry about that sort of thing and.. just enjoy yourself (faces audience) that goes for you all too."
Austin: (also facing audience) "Yes"
The reason the universe is ending in Star Crossed is because of all the various paradoxes created by the Federation, from Jim Kirk to Captain Janeway.
In Strange Attractors by William Sleator, almost any time travel to the past causes instability in the universe. As those instabilities add up, the entire universe can "go chaotic", essentially becoming a huge mass of paradoxes. The only noticeable effect of this is that electrical lighting flickers. In fact the timeline in the series is so fragile you can cause paradoxes by going so much as five minutes into the future.
In Ted Chiang's short story The Merchant and the Alchemist's Gate, the titular gate can transport anyone exactly twenty years into the future, or twenty years back. This leads to increasingly more improbable shenanigans, starting with a Stable Time Loop involving a treasure map, and reaching its arguable peak when a character's wife meets her husband's younger self in the past, takes him to the bedroom, and upon descovering his lack of the, er, skills that the husband has in the present, teaches him how to please a woman, over the course of weeks. It's also implied that the husband married her in the present because, when he saw her, she reminded him of the middle-aged woman who took his virginity.
In L. Sprague de Camp's short story "A Gun for Dinosaur," four characters (two hunters and two guides) travel to the Cretaceous period for a dinosaur-hunting safari. One of the hunters, Holtzinger, is killed by a tyrannosaur, and the other, James, is blamed for his death because he recklessly fired the shots that woke the dinosaur up. Later, James, swearing revenge, tries to go back to just before the expedition arrived so he can kill the guides once they emerge from the time machine. Instead, the space-time continuum snaps him back to the present to prevent a paradox, killing him messily.
The novel Ice And Blood manages to produce a very strange reverse grandfather paradox. ZJ is a depressed, bipolar paranoid schizophrenic who has no memories of his childhood. He hates his life enough to deliberately break into a lab where time travel technology exists, he goes back to the past, and he kills his parents in the hopes he'll stop existing. Instead the violent and bloody deaths they suffer triggers his past self's mental illness. The obvious problem with this is that there's no guarantee that ZJ would suffer the exact same breakdown and block out his memories every time, nor is there any logical reason that depressed ZJ would ever go this route again when suicide would be significantly easier for him. It just doesn't work from a logic standpoint. (It's still a good read if you apply enough MST3K Mantra to it.)
Live Action TV
When, in the Red Dwarf episode "Tikka To Ride", Lister attempts to explain the temporal paradox of the last season finale, the camera explodes. Repeatedly.
In the Star Trekuniverse, time travelers (and the writers) are generally immune to the effects of changes they make to the timeline, and can therefore find themselves in an Alternate Universe where they should not exist (as in "The City on the Edge of Forever" (TOS), "Yesterday's Enterprise" (TNG), or "Cold Front" (Enterprise)).
Star Trek: Deep Space Nine has an episode with Chief Miles O'Brien going forward in time a few hours and then, when he feels he's about to die, sends his future self in the past to take his place and prevent the disaster.
Chief Miles O'Brien and Chief Miles O'Brien: I hate temporal mechanics.
One way this could work is that the original Present Miles was from and the Past he goes to have become Alternate Universes. The "first" universe, what was called the Present, Miles would never return to. However, the show just continues in the changed Past universe.
And in ""Trials and Tribbleations":
Lucsly: So you're not contending it was a pre-destination paradox?
Dulmer: A time loop? That you were meant to go into the past?
Sisko: Um... no.
Lucsly: We hate those.
Narrowly averted in the episodes "Past Tense I & II", where-in Sisko successfully impersonates a historical figure after the man is killed (saving Sisko's life no less) before he can make the Heroic Sacrifice for which history remembers him. That Sisko bears an uncanny resemblance to historical record of the man's appearance becomes something of a Brick Joke in later episodes.
Deep Space Nine had plenty of other time-travel episodes, mainly due to the presence and involvement of the Wormhole Aliens/Prophets of Bajor who existed outside of time preventing any danger from a paradox. The hand-wave itself is even lampshaded in the resolution of "Accession"
In the Time Travel RPG Continuum, if a time traveler creates a paradox, they accumulate "frag," and if they accumulate too much, it eventually causes them to unravel. What's more, unchecked temporal paradoxes will eventually lead to the unraveling of reality itself. (On the other hand, the game totally mindscrews you with the fact it manages to be utterly fatalistic about it: the fact the universe itself exists, even if there is a paradox in existence, means that — at some point — the temporal paradox will be/does get/has been fixed, by the Continuum. It's just a matter of who or what becomes collateral damage in the process of fixing it.) Much of the game centers around the players, who are part of "The Continuum", trying to fix paradoxes deliberately created by time travelers (known as "narcissists") who don't believe the official line on paradoxes and who want to mess with the timeline for their own personal gain.
Similarly, the expansion sourcebook (currently trapped in Development Hell) Narcissist has a different take on this — the original time traveler entered the "main" timeline's past and introduced time travel sometime around 14000 BC. Said time travel directly resulted in a singularity around 2400 AD, which then used its super-powerful minds and infinite resources to make sure that said time traveler never leaves our timeline (which would require a portal made out of X number of Temporal Paradoxes), and that time travelers don't cause the timeline to deviate from the history that led to the singularity. In alternate timelines away from "the swarm" — agents of the Singularity, named that because there's a lot of them, but they're disorganized idiots — paradoxes don't exist: "frag" exists in the main timeline specifically due to the singularity's agents constantly trying to time-Mind Rape anyone attempting to change history.
Averted in Genius The Transgression. As the game puts it, it turns out the universe doesn't particularly care if your grandmother gets shot and there's no shooter — barring external intervention, you pop out of existence if you pull the trigger and the bullet hits home. This can have some interesting consequences, as the angry young lad seeking to avert a massacre in his country's history did not discover...
In the RTS Achron, you can use the free form time travel to create some interesting paradoxes. Apart from the Grandfather Paradox and the Ontological Paradox in many different incarnations, you can create a stable feedback loop to continuously strengthen your army. For example, if 10% of your army survive an attack, you can send these back in time to support their past selves in battle. That way, more units will survive and more units will get sent back in time. Thus, more units will survive. It can also happen the other way around, with your army getting continuously weaker, but that is much rarer and harder to spot. For example if the units you send back in time chronofrag (similar to Tele-Frag) their past selves. Since all players have these abilities, results can get quite unpredictable.
In Chrono Trigger one character accidentally paradoxes herself out of existence, forcing you to set things right before history notices. Later you get to do things like take an item from a treasure chest and then go back in time four hundred years and loot its (inferior) temporal duplicate, or in the DS remake combine an object with itself. The characters in-game speculate that some sort of "Entity" or outside force is responsible for keeping all this from getting out of hand, which is convenient.
Chrono Cross is a particularly confused case, involving as it does four, maybe five different factions manipulating an Unwitting Pawn across two dimensions. The Chronopolis research facility attempted a time experiment that ended in a Time Crash, hurling it back in time about fourteen thousand years and forcing it to carefully manage history to avoid any paradox that would prevent its future existence. But then someone meddled in a pivotal event and split the timeline in two, so that in one dimension Chronopolis still exists, while in the other a time-frozen slice of Chrono Trigger's Bad Future has come to fill the void. Also, this may unleash a force that could destroy all of existence.
Ratchet & Clank Future: A Crack In Time's main focus is the Great Clock, which, originally designed to keep time, is being sought out by the villains to ALTER time. Orvus states this would create multiple paradoxes and blow up the universe.
In Phantasia, it works like this: Trinicus Morrison sends Cress and Mint 100 years back in time to acquire the power (magic) needed to defeat Dhaos, since they couldn't get it in the present (nevermind the fact that Dhaos was literally about to waste the lot of them with his lazers). In the past it turns out that the reason magic didn't exist in the "present" was because its source withered and died, so it's less "get magic" more "keep magic from dying". They succeed but wind up getting drafted to help out in a war against Dhaos, and during said war Edward Morrison (ancestor to the guy that sent you back in the first place) sacrifices himself to prove to one of your party members that Dhaos's methods are evil. Problem is, according to history and the opening cutscene-battle-thing Edward was one of the people who fought Dhaos in the past and forced him to flee to the "present" where he got sealed; with him dead, history would be pretty much fcuk'd, so the party takes it upon themselves to engage Dhaos in the past (with Arche taking the place of Edward and Cress and Mint taking the places of their respective ancestors). Dhaos flees through time like he was supposed to after that, and after finding the time machine the party returns to the "present" at the point right after Trinicus sent Cress and Mint back twenty-seven hours ago.
Time Hollow for the DS subverts these, for the most part, by having a few people remember all alterations - as such, you can't hit the "prevent myself from adjusting time" snag by fixing the thing you wanted to change. There is, however, of all things, a cat that ends up in a near-ontological paradox avoided only by the fact that it's locked in time and cannot age.
Super Robot Wars Reversal's plot. In the future, the world is turned into a Crapsack World. Chance encounter with Duminuss causes Raul/Fiona to be thrown back to the past, before the world goes gloomy. At that point, he/she decides to screw the bad future and make paradoxes here and there, incidentally making the future brighter when they come back in their own time.
Ecco the Dolphin 's time paradox consisted of Ecco seeing the Asterite, who tells him to go back in time and recover its missing globe from the Asterite of the past. That means that in the past, a dolphin stole the Asterite's globe, and only because of a request from the future.
As part of its recurring themes of You Can't Fight Fate and Screw Destiny, the Legacy of Kain series establishes that time travel always creates a Stable Time Loop, where if you try to change history on your own, the timestream just re-establishes itself as it is meant to be as though you weren't even there, like a river flowing around a stone. The only way to truly alter time is to cause a paradox by bringing the same object or person from one time into close proximity with itself in a completely different time. This creates a disruption in the timeline strong enough to alter history in ways that cannot be predicted, the butterfly effect occurring and history shifting itself around to fit the new chain of events. The main item for this task is the Soul Reaver, due to it being wielded by a variety of powerful figures including the two protagonists, and thus bringing two swords into contact means someone of historical importance is going to die.
This mechanic is actually the crux of the entire plot. Because time travel always creates a Stable Time Loop, it means free will doesn't exist, You Already Changed The Past and You Can't Fight Fate—or so it seems. The exception to those rules is the paradox as explained above, and it just so happens that the spectral version of the Soul Reaver that Raziel wields is actually his own soul. He receives the sword in the present, and will travel back in time where he will be drawn into the material version of the Soul Reaver in the past, thereby becoming its spectral half to one day be bonded to him when Kain destroys the material version attempting to kill him. Raziel is walking around with his own soul clinging to his arm, and thus he's a walking paradox, the only person with free will that has the power to change history freely because he creates a temporal distortion wherever he goes. This is why so many people go to such extreme lengths to manipulate Raziel into changing history for them.
At one time there's actually a four-way paradox - during the climax of Defiance, you've got Past Kain wielding the Soul Reaver that belongs in this time, Present Kain traveled back in time with the original Reaver taken from a point in time further in the past before it became the Soul Reaver, Raziel traveled back in time with the spectral version of the Soul Reaver that was created when the material Soul Reaver currently wielded by Past Kain was destroyed in the future, and Raziel himself who will eventually be drawn into the Reaver wielded by Present Kain to become its spectral half and transform it into the Soul Reaver. Oh, and the Reaver that Present Kain is wielding will also have to be taken back in time at some point after absorbing Raziel's soul so he can leave it for his past self to find and thus leave the timeline up to that point intact.Got all that?
Jak and Daxter: At the end of Jak and Daxter: The Precursor Legacy Jak and company find a "rift rider" lying around together with a huge portal. Everyone gets in; they are sent to the future and land in a dystopian city. Keira and her father get away, but Jak is captured and the rift rider is destroyed. Two years pass. Jak II: Renegade starts. When Jak meets up with Keira again, she's working on another rift rider, trying to recreate the one they found from memory. During the game, they meet up with a kid. Eventually Keira finishes the rift rider so they can go home. But it turns out that the Kid is a young Jak, who was born in the future. So Samos has to take him on the rift rider back to the past where he can be raised safe from harm and come back to the future to fulfill his destiny. It turns out that the rift rider Keira built from memory wasn't a replica, but the very same one they found in the past. So they have to go to the past to drop off the rift rider, so past Jak and company can find it and then go through the rift back to the present and— Oh no, I've gone cross-eyed.
Daxter sums it up for us:
(to a very confused Keira) Honey, the more you think about it, the more it hurts the head!
Radiant Historia has them. Logical, since one of its central themes is Time Travel. One of the most obvious examples is a mission where you talk to a grieving widow, who laments the medicine she got for her husband never arrived till it was too late, leading the party's hero to accept the medicine and give it to the man in the past, causing him to feel better, negating the need for ordering the medicine in the first place. Well, gosh.
Stocke is explicitly told early on (though in not as many words) that as the wielder of the White Chronicle, he's a walking blind spot in causality, able to experiment with the timeline paradox-free. Since at least a thousand years of history is littered with Chronicle wielders jumping around to visit key points in their own lifetime that never happened the first time around without the time stream collapsing like an accordion, this is at least internally consistent.
The Czech-developed RTS game Original War revolves completely around this. Twenty Minutes into the Future, just as the world's oil reserves are becoming exhausted, the mineral 'Siberite' that enables Cold Fussion is discovered in Siberia, giving the Russians a monopoly on the new energy source. In order to avoid Russian dominance, the USA uses an alien time machine found during a World War One expedition to Siberia to send volunteers in a one-way voyage to the early Pleistocene, the only period the machine can be set to, and use them to mine and move all the reserves of Siberite to will-be Alaska over a land bridge connecting the two regions at that time. Once there, however, they find Soviet troops from an alternate timeline ready to fight them. The A-Soviets also found an alien time machine in a Siberian archaeological site, along with rests of mining machinery and traces of Alaskite, a mineral that enables Cold Fussion that is only found in Alaska and is threatening to give the Americans the world's energy monopoly, so they used the time machine to send volunteers to the early Pleistocene and move all the reserves of Alaskite from will-be Alaska to Siberia...
The Into The Future expansion of The Sims 3, allows you to go into the future. However, by going there, you risk creating one of these, which could erase the sim in question from existence.
Breakpoint City has several examples. Some are played straight, some are... not. A few have been explained away as due to alternate universes, therefore probably not a paradox, but you never know.
This is confusing. How is a flashback to the childhoods of the Cheer girls even possible? Weren't they, you know, boys? Just how much of the past did Miranda rewrite to cover up Anne's mistakes? Is it like what happens when a misfile occurs? Argh...maybe it's best to pretend this isn't canon, especially seeing as there are lots ofpeoplewho still remember.
Well, three of the girls do not remember ever being anything but girls, so presumably their memories were altered. As for Jo...
In the Surreptitious Machinations arc of General Protection Fault, Empress Trudy travels back in time to give her younger self the necessary information on what she must do to take over the world. Near the end of the arc, Nick and Ki's son Todd reveals that the entire Bad Future he and Empress Trudy came from was the product of a temporal paradox, since it could not have happened without Empress Trudy advising her younger self, which would not be possible if it did not previously exist. It is heavily implied that Pandemonium was responsible for the existence of the alternate future in the first place. As a result of the future being changed, Todd, the Empress and all other objects from the alternate future fade from existence, but the Empress teleports to a different time just before she fades, and the Gamester finds and recruits Todd.
ThisRock, Paper, Cynic comic. A man uses an axe to murder someone and get at a time machine. He then briefly experiments with it before realizing that he doesn't know where his alternate self that should exist is. Then, by intention or accident, he goes back to the moment first chronicled in the comic, where he becomes the man his past self killed to get at the time machine.
The end of The Onion article "Pistorius Case Takes Dramatic Turn As Altered Plane Of Reality Results In Paralympian Shooting John Lennon": At press time in November of 1986, South African officials reported they have apprehended a newborn Oscar Pistorius for the murder of John Lennon, thereby preventing the deaths of both Reeva Steenkamp and John Lennon at the hands of a 26-year-old Oscar Pistorius, and thereby making Pistorius a free man.
The Fairly OddParents special, "The Secret Origin of Denzel Crocker". Timmy goes back in time to find out why Crocker was so miserable and to try to fix it. He finds out that as a child, Crocker himself had fairy godparents—and that they were Cosmo and Wanda, something that they don't remember—and figures out that he must've done something to lose his fairies. He tries to warn the young Crocker, but inadvertently ends up being the one who reveals the secret (with some help from both '70s Cosmo and modern Cosmo's stupidity). Furthermore, as Jorgen shows up to erase everyone's memories of there being fairies, young Crocker manages to get his hands on the DNA tracker that AJ had built so that they'd know when Crocker was around, and managed to get Cosmo's DNA to use in it, and managed to covertly write a memo on the back of it that fairy godparents exist without Jorgen noticing, allowing him to keep that knowledge after his memory of fairies was erased...which means that if Timmy had never interfered, Crocker would be neither miserable nor fairy-obsessed. However, whereas when Timmy left for the past, Crocker was using a very primitive and likely useless "fairy finder", the Crocker in the present that Timmy returned to was using the tracker that AJ had built, implying that he had created an alternate timeline, and leaving one to wonder what happened in the original timeline. Of course, considering it's explicitly stated in The Movie that few kids keep their fairies past their first year, much less until adulthood when they would leave anyway, we can guess...
Well the original timeline seems to be that 70's Cosmo is that cause of Crocker losing him and Wanda. Timmy then stops this incident only for present day Cosmo to turn on the mic while Timmy is talking and cause the incident to happen anyway. While this doesn't explain how Crocker knew about the existence of fairies after his mind was wiped in the original timeline, since we don't see the original incident play out, we can just assume any number of reasons for that. (Perhaps he managed to write a note in that timeline too.)
Lord Chronos was at first a meek scientist who invented time travel. He used it to steal things from history that would not affect the timeline. Then his wife nagged him about his lack of imagination, and one trip to the Wild West later he decided that stealing the most famous items from history and setting himself up as master of space and time was the better way to go. Reality itself falls apart, so he decides to go to the beginning of time and do it all over again. Batman and Green Lantern manage to reset history. Batman also manages to create a close And I Must Scream moment by trapping him in an eternal loop of his wife's naggery.
Time travel itself seems perfectly fine: Superman went to the future and the past, the Justice League went to WWII, one of the team is from the future, and the list goes on. Vandal Savage notes that his time machine couldn't send him back to a period where he already existed, which may be with good reason: in that episode, time was falling apart only when Batman was with his very old self.