Temporal Paradox



""History abhors a paradox.""

- Legacy Of Kain: Soul Reaver

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 violating this rule may 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 unleash killer 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:

Grandfather Paradox:

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.

Ontological Paradox

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 ways. 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 you 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 (e.g. 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 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 completely Set 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.

Anime and Manga

 * Toriyama follows the multiverse approach in Dragonball 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.

Comic 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 is true.
 * This is generally accepted; however, it has been shown that Dr. 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.
 * 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."

Film

 * Pretty much every film in the Planet of the Apes franchise.
 * In the Back to The Future movies, Doc Brown is very concerned with paradoxes. Paradoxen. Paradoci? [PARADOCES]
 * 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.
 * The Lake House is a story about a mailbox that delivers letters from Kate to Alex two years ago and vice versa. It should be noted that the Korean movie this movie is based on dealt with the paradox differently:
 * 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.
 * 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. This causes everything that was changed by his time travel to exist outside of the normal timeline without affecting it. Maybe.
 * , which makes the whole thing a massive paradox anyway.
 * Even worse (better?) is that
 * The whole plot of the movie Butterfly Effect revolves around the main character's ability to travel back in time and change parts of his life. Every change causes
 * Meet the Robinsons. Let's see. If Goob made the catch and won, getting himself adopted and never becoming, 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 film The Time Machine, 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.
 * I still wonder why he couldn't use this knowledge to travel back (The Hannibal gave him a chance to do that), save the lass and remember to construct that machine, so he'll travel back and forth during some point in the future. If you argue that changing the past in this fasion wouldn't make a time loop I say that protagonist changed the future after confronting morlocks because he saw it. If he changed the future, then he wouldn't be able to see that grimdark landscape that motivated him to change time - no paradox here. Of course, maybe he didn't stopped the underground from enslaving humanity after all..
 * 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...

Literature

 * Lazarus Long, protagonist of Robert 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.
 * In The Dark Tower, Roland kills the man who murdered Jake, who Roland met in The Dark Tower. He spends the first part of The Dark Tower fighting off insanity because of the paradox this creates.
 * To say nothing about what happens to Jake in the first part of The Dark Tower who is both alive and dead at the same time.
 * The The Caretaker Trilogy 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 occuring. 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.
 * Some argue that René Barjavel's Le Voyageur imprudent is the first ever example of the grandfather paradox.
 * 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 —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
 * Distilled to its purest form in Fredric Brown's short story Experiment.
 * 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 Hitch Hikers 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 occuring in the future.

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" had 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 killer flying time monkeys, 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  and with everything in that folder she handed the Doctor, the transcript, several pictures of Angel statues, the list... I wonder where those scavenger Weeping Angels came from anyway.
 * It gets much weirder in the Doctor Who Expanded Universe, which features Faction Paradox, a villain group whose hat is temporal paradoxes. In fact, part of their initiation ritual involves traveling back in time and killing off your own ancestors. Yes, really.
 * 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!
 * And that's before you enter the Eleven-Day Empire, a place literally made of nonexistant time. Or the Grandfather Paradox, the Anthropomorphic Personification of all potential evil and despair in the Universe. Or the part where Gallifrey's history is repeteadly raped into oblivion.
 * 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".
 * 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, 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 . So, it's not that Valen's soul went into that man, but that . 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 ), which would...yeah.
 * Actually, a similar instance can be seen in the Haruhi Suzumiya anime and light novel series. In the "Endless Eight" story arc the central characters, . There is also an episode of Star Trek: The Next Generation which deals with a very similar theme. So it could theoretically be argued that the Minbari did choose thousands of different human pilots at thousands of different Battles of the Line, before they finally chose Sinclair. Although it seems far much more likely, or at least less complicated, to assume that Sinclair.
 * 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.

Tabletop Games

 * 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 the 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.

Video Games

 * In Achron paradoxes are a deterministic and fully logical gameplay element. So pretty much a Aversion on the illogical/indefinite portion of the trope.
 * The Metal Gear Solid games are well known for situations occuring 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 Nonstandard Game Over. It's especially surreal when your CO from the future starts chewing you out for causing a Time Paradox. The fact that EVA (who is never mentioned in any other game in the series AFAIK) is protected by a similar Nonstandard Game Over adds an additional layer of surreality.
 * She's since appeared in 2 more. They were just giving her a paradox vaccine.
 * And, in a more immediate matter, she  As such, she has to make it through to the end of the game.
 * 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, you get a slightly different Have a Nice Death message.
 * Infinity series:
 * At one point, Ever 17 features a time-travel attempt to Set Right What Once Went Wrong with an obvious fix. Unfortunately, this also undoes the events that triggered the attempt, resulting in [[media:ever17 time paradox.jpg|a very unpleasant paradox]]. Warning. That link is hellishly disturbing.
 * In several Bad Ends of Remember 11, death of one of the main characters causes the story's Stable Time Loop to shatter, leading to them discovering themselves sometime before the exchanges started -- for Kokoro, and  for Satoru.
 * Time-travelling in miniature into your own brain can have equally unpleasant results in The Hitchhikers Guide to The Galaxy Infocom text adventure. When the minaturisation wears off, you cause the head you're in to explode. You're fine... but you won't be when the minature you, now inside your own head, expands to full-size...
 * 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, the Temporal Paradox does have quite a devastating effect -  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 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.
 * TimeShift 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 crazy shit, such as reversing time and shooting from two seperate 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 Infocom Interactive Fiction game Trinity combines this with a Stable Time Loop:
 * 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 with immunity from changes in the timeline granted by
 * The Ubisoft Prince of Persia trilogy is entirely based around this concept.
 * In the first game 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
 * Cue the second game, 7 years later.
 * Which takes us to the third game.
 * 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 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.
 * 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 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.

Visual Novels

 * The actual objective of Servant Archer in Fate/stay night.
 * Besides,

Web Comics
"Imagine... enjoying your favourite sandwich... then going back and stealing the sandwich from yourself before you eat it! So you get two sandwiches."
 * 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:


 * 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 of having an unworkable plan because of a variant on this..
 * Reaction:
 * of The Adventures of Dr. McNinja traveled back from the future to stop a Zombie Apocalypse. Then another of him showed up in order to stop a Robot Uprising. And then more and more.
 * The high cost to reality is invoked and elegantly resolved in this Penny Arcade strip.
 * In American Barbarian, Rick refuses to do anything that would prevent their births.
 * 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.
 * 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.
 * 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.

Western Animation
"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)"
 * The Danny Phantom movie The 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. "Don't you get it? 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."
 * Naturally enough, the animated series of Bill and Teds Excellent Adventure often courted this trope. One obvious example is the episode in which Bill and Ted neglect to buy Bill's father an antique railroad watch as a birthday present, to replace the one he lost as a child. Ted's initial plan is actually perfectly sound: take the original watch from Bill's father when he 'loses' it in the past, then give it to him in the present. This plan fails however, so they travel even further back in time to obtain the watch before Bill's father inherits it. Of course, this should mean that Bill's father wouldn't miss the watch in the first place, but the episode simply ignores this.
 * 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:


 * Superfriends. In the Challenge episode, "Secret Origins Of The Super Friends," the Legion of Doom tries to change history by messing with the origins with 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?
 * 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?
 * No, the real question is: Why does the Legion of Doom use Time Travel for such petty activities as foiling the origins of 3 of their 11 nemeses, or stealing a few ancient trinkets? That's like using a tactical nuclear weapon to open a can of peas.

Anime and Manga

 * In The Disappearance of Haruhi Suzumiya,  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 ). They finally.
 * 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.  In the Episode of Bardock spinoff it turns out that Bardock   He fights Chilled, , and during the fight he   This means that Bardock   That in turn means that
 * What's more is that Freeza

Comic Books

 * 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 . 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,
 * 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.

Fan Works

 * Link saving himself in The Legend of Zelda: The Return.

Film
"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."
 * Another Sandra Bullock film, Premonition, mixes this trope with You Can't Fight Fate: which she could not have done had she not had the premonitions of the future.
 * Referenced in Deja Vu by agent Carlin right before they send a note back in time:

"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."
 * 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.
 * Actually, the premeditated part was the fact that Anderton had spent years thinking about how  This leads the antagonist to set up a situation where
 * 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 2002 remake of The Time Machine, 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 in Star Trek IV: The Voyage Home when Kirk pawns his reading glasses in 20th Century San Fransisco.


 * Thinking about that a bit more suggests the possibility that said glasses were switched out somewhere between selling them to the pawn shop and giving them to Kirk, so the loop wouldn't be closed. It's not like there were no cataclysmic events like World War III in between.
 * Even better the gift again could be a reference to the cash he receives from selling them. Considering the original pair had the lenses intact and the one sold in the pawn shop did not, it is likely the ones sold in the pawn shop did not originate from there.
 * 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.
 * 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
 * Since the, 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 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  to the island, even though she knew full well that she would be the one to
 * 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 . This means that 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.
 * 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.

Literature

 * 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--,
 * 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.
 * In Artemis Fowl and the Time Paradox,
 * Not to mention,
 * 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).
 * But by the nature of the series, couldn't someone just enter H. G. Wells's novel and bring out a working time machine?
 * In Harry Potter and the Prisoner of Azkaban,
 * 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.
 * in Pyramids, with

Newspaper Comics

 * Unsuccessfully 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 went to the future to get it two hours ago. 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
 * The timeline of this whole thing is a little paradoxical, but at least the object/information obtained has an origin.

Video Games

 * 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.
 * It is also possible to make a unit literally become its own grandparent/grandchild.
 * 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.
 * Now the real question is, where did the Song of Storms originate?
 * To even further compound the Temporal Paradox, 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...
 * 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...
 * 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 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 power and 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.
 * 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.
 * 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.
 * In |Sonic the Hedgehog 2006, what happened to the Blue Chaos Emerald? Silver gives it to Elise, then it goes missing, Silver and Shadow escape from Radical Train, and the cycle closes because they then give it to her.
 * In this same game, 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:
 * Ever 17: Information Loop in
 * Remember 11: 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

Web Comics

 * At the climax of the Doc Gets Rad chapter of The Adventures of Dr. McNinja, villain 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
 * In this Faulty 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:
 * It should be noted that those examples happen all the time. Locally to John's session
 * 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.
 * SSDD, 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.

Web Original
"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.""
 * 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.
 * He died from the heart medicine Church gave him to prevent that.
 * The tank's AI named Sheila?


 * Guess who changed the default setting.

Western Animation

 * In the first Futurama movie, the "paradox-free time travel" isn't quite paradox-free: From whence did the name come? No one knows.
 * I do. Oh, no. I've gone cross-eyed.
 * Well, if we just accept that  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, the question is where exactly the information carried on his y-chromosome originated.
 * 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,
 * Gargoyles: Time travel always creates a Stable Time Loop:
 * David Xanatos planned 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 Goliath sends it to the distant past to escape Puck's grasp and eventually gets discovered in the first place. The loop is looped.
 * 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 insiginificant 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.
 * 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.

Fan Works

 * 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 24. 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.

Film
"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""
 * 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...
 * Deja Vu contradicted itself on terms of Temporal Paradox. 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, but in the end, it is contradicted, because
 * 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


 * 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.

Literature

 * 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
 * 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
"Chief Miles O'Brien and Chief Miles O'Brien: I hate temporal mechanics."
 * In the Star Trek universe, 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.

"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. Dulmer: Good. Lucsly: We hate those."
 * 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":


 * 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"
 * 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.

Tabletop Games

 * 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. 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 2500 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.
 * Time Travel is rare in Warhammer 40,000, but the Warp does strange things sometimes, like sending ships off to answer their own distress signals. In another example, one Ork Warboss was sent back through time via warp-storm, met up with his past self, and killed his temporal doppelganger so he could have two copies of his favorite gun. The resulting confusion stopped the Waaagh! in its tracks.
 * 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 notdiscover...
 * In Time and Temp, a paradox would Ret-Gone all of existence. Office temps (hence the name of the game) are used as field agents to prevent this, because they're otherwise unimportant enough to minimize the risk of personal Grandfather Paradox - though their potential for incompetence is at odds with this.
 * The GURPS Sourcebook GURPS Infinite Worlds includes a chapter exploring time travel and paradoxes.

Video Games
" Daxter: Honey, the more you think about it, the more it hurts the head!"
 * 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 put history back on track. Later you get to do things like take an item from a treasure chest, then go back in time four hundred years and loot its (inferior) temporal duplicate.
 * It is implied ingame, by the main characters no less, that there was an 'entity' or outside force making them travel through time. This means it's less of a headache, if someone with UBER POWER is driving the time bus.
 * The DS remake manages to make it even worse. There's now a sidequest that involves combining an object with itself. Causality is Crono's bitch.
 * Chrono Cross. Like Chrono Trigger, Only More So: the party themselves aren't time-traveling, instead serving as the pawns of four, maybe five different factions manipulating history for their own ends. Oh, and there's also an alternate timeline involved as well.
 * In Ratchet and Clank Future: A Crack In Time's main focus is the Great Clock, which, originally designed to keep time,
 * Tales of Phantasia and its sequel Narakiri Dungeon 1 are even more confusing.
 * 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 that ends up in a near-ontological paradox avoided only by the fact that
 * 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 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 Got all that?
 * Jak 2: Try to keep up: Wrench Wench Keira found a "rift rider" (basically a time machine) just lying around (just go with it). So everyone gets in, flies 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. 2 years passes. The game starts. When you meet up with her again, she's working on another time machine, trying to recreate the one they found from memory. During the game, they meet up with a kid who looks a lot like Jak. At the end of the game, Keira finishes the rift rider so they can go home. But they have to take the kid with them who turns out to be . So they have to take him on the rift rider back to the past so he can and fulfil his destiny, because 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 Keira 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.


 * |Sonic the Hedgehog 2006 is bnuh guh nyuh gubuh buh... blark... ARGH! Thank God for the
 * Mephiles hates Shadow for sealing him, and attacks when he's freed. Shadow hates Mephiles for attacking him, and seals him. What?!?
 * Radiant Historia has them. Logical, since one of its central themes is Time Travel. One of the most obvious examples is . Well, gosh.

Web Comics

 * Super Stupor's Clockstopper can change history with his "Time Punch". (And he'd rather be surfing TVTropes than fighting crime.)
 * 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 of people who 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 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.
 * This Rock, Paper, Cynic comic. Wait, what happened?
 * It's pretty simple. 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.
 * Bob and George Information ontological paradox

Western Animation

 * 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 explcicitly 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)
 * There was also a Historical In-Joke to imply that it was an alternate timeline.
 * In Justice League, 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's decided that stealing the most famous items from history and setting himself up as master of space and time is 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 seams perfectly fine, Superman went to the future and the past, the Justice League went to WW 2, one of the team is from the future, the list goes on. Victor Strange notes that one cannot be in the same time period as one already was in. One should note that in above mentioned episode, time only was falling apart when Batman was with his very old self.