->''"Alright, this next test may involve trace amounts of TimeTravel. So, word of advice: if you meet yourself on the testing track, don't make eye contact. Lab boys tell me that'll wipe out time. Entirely. Forward and backward. So do both of yourselves a favor and just let that handsome devil go about his business."''
-->-- '''Cave Johnson''', Aperture Science CEO, ''VideoGame/{{Portal 2}}''

For some reason, encountering yourself--whether as a [[TimeTravel time-traveler]] or in AnotherDimension--is frequently Very Bad; either it's unhealthy for you in particular, or it creates a TemporalParadox. While this is frequently incredibly useful as a plot device, it doesn't make sense; if there are negative effects to existing simultaneously with yourself, why should they depend on proximity? And why would it require that you as a unit be next to yourself, rather than, say, the atoms in your body (of which there may be few left if you meet yourself as a baby)?

This may result from attempts to apply causality to time travel: you can't meet a past version of yourself without having memory of it and the future version of yourself cannot be surprised to see past you, having had to be there to see it to make it possible. However, this can be easily avoided if you fail to recognize yourself. As a result it's generally correlated with time travel of the TimeyWimeyBall variety, and negatively correlated with {{Stable Time Loop}}s (though there are exceptions). Compare FutureMeScaresMe; contrast ScrewYourself.

Note that this trope is now so well known that movies which use Time travel often only throw out a one-liner warning: "Whatever you do, don't meet up with yourself!" It's now becoming a trope in and of itself to make the reason for not meeting up not a dire end-of-the-world reason, but merely for convenience of the Master Plan. These often result in MyFutureSelfAndMe.

A closely related trope is OnlyOneMeAllowedRightNow.


[[folder:{{Anime}} and {{Manga}}]]
* In ''SuzumiyaHaruhi'', future selves take great care to conceal themselves from past selves who didn't see their own future selves. However, they also take just as much take care to '''reveal''' themselves to past selves who remember seeing or hearing their future selves. Basically, future selves act so that [[StableTimeLoop their past self experiences events as their future self remembers them.]]
** Future Asahina conceals herself from her past self almost every single time, with the explanation that her past self didn't see her future self. The one exception is when [[spoiler:Kyon, Asahina (small) and Nagato go back to December 18th to save the world; Asahina (small) sees Asahina (big), but doesn't recognize her, and doesn't pay her much attention. There are slightly more important happenings going on, like, y'know, ''Kyon bleeding to death in the middle of the street.'']].
** In ''The Disappearance of Haruhi Suzumiya'' we see Present Kyon hide so as not to reveal himself to Past Kyon from "Bamboo Leaf Rhapsody," who he remembers didn't see him; but towards the end, [[spoiler:when Kyon gets stabbed, his future self arrives and tells him some words before he passes out. After he wakes up, Kyon recognizes that the voice he heard was his future self, and that when he travels back in time to that day again, he must tell those words to his past self.]]
* In the manga version of ''Manga/SailorMoon'', during the TimeTravel to the thirtieth century, the titular character feels faint in the presence of her future self, Neo-Queen Serenity, and her body becomes transparent; her Silver Crystal also loses its power under the influence of its future counterpart. At the end of this story arc, Neo-Queen Serenity tries to resist the temptation of talking to her past self, since it may result in the history being changed... fails, and goes to meet Sailor Moon anyway. The story also involved Prince Demand attempting to [[TheEndOfTheWorldAsWeKnowIt bring the world to an end]] by bringing together the two Silver Crystals. Interestingly, in ''Stars'' it appears that the selves ''can'' meet, if one of them is (supposedly) from an alternate distant future and in disguise. Possibly.
* Played straight in KimagureOrangeRoad. Touching your past self will cause the future version to dissolve. Never really explained exactly what happens to the future self but it gets shelved under "a bad idea to try".
* Played with quite a bit in ''Manga/MahouSenseiNegima'' during the Mahorafest arc. The aim is to not let the selves meet, but often the later selves run into the earlier selves, and at times, a character runs into one version of a character almost immediately after leaving another version of the character, causing confusion--especially when it happens with one of the girls ''not'' in on the {{Masquerade}}. Also, at one point, we distinctly see two Chamo-kuns meeting up and high-fiving. And then ''switching places!'' How that's supposed to work is [[MindScrew mind-boggling]]
* ''Manga/NatsuNoArashi'' firmly enforces this. People start to fade when they get close to their past selves, and if you meet yourself, you'll vanish entirely. Time doesn't like people having two of themselves in one place.
* In ''{{Kurokami}}'', every person has two dopplegangers. When they meet two of the three wither away, but the third will thrive at the cost of the others.
* In ''Manga/KatekyoHitmanReborn'' the past and future versions of the Vongola are not able to coexist in the same timeline.
** Actually, at the end of the [[spoiler:TYL!]] arc, we find out that [[spoiler: the TYL! Vongola guardians are being kept in some weird capsule thingy but do see/talk to their past selves.]] Although they were admittedly in what could have been a [[spoiler: separate pocket dimension created by the current BigBad 's men]].
** An attempt to [[DefiedTrope defy]] this trope [[GoneHorriblyWrong gone horribly wrong]] resulted in [[spoiler:Ghost, who was a version of [[BigBad Byakuran]] from an alternate timeline that was brought over to the main timeline. In the process, Ghost's own [[AlternateUniverse parallel world]] was destroyed, and Ghost himself was left as a [[EnergyAbsorption Flame-absorbing]] EldritchAbomination with hardly anything resembling a [[AndIMustScream human consciousness]].]]
* ''SeikimatsuOccultGakuin'' goes for the worst possible variation. [[spoiler:Fumiaki meeting his past self overloads the timeline and causes the apocalypse. Though he does ScrewDestiny and prevents the thing he caused.]]
* In the ''Manga/JoJosBizarreAdventure'' story arc ''Steel Ball Run'', this trope is applied to versions of people and objects from parallel universes. In "Dirty Deeds Done Dirt Cheap", the [[PsychicPowers Stand]] of the BigBad [[spoiler: [[PresidentEvil Funny Valentine]]]], specifically protects him from the effects of this, but also allows him to use it against his enemies.
** Even after his defeat, the final enemy ([[spoiler: an alternate Di(eg)o Brando]]) still meets his end through this rule [[spoiler: courtesy of Lucy Steel and the decapitated head of the main universe Diego]].
** This is actually used in the ''All-Star Battle'' video game as the character's strongest attack - he summons a version of his opponent from another universe, and then throws the two of them into each other, causing both to detonate.

[[folder:Comic Books]]
* An old SilverAge story involves Wonder Girl (WonderWoman as a girl, before she became a separate character) attempting to meet her adult self but failing; this is the reason given for the failure. Later, it's why there needs to be a HandWave ("Amazon magic") for why Wonder Girl and Wonder Tot (Wonder Woman as a toddler) are able to team up with the adult Wonder Woman.
* Other [[CrisisOnInfiniteEarths Pre-Crisis]] DCComics stories, most notably in the ''{{Superman}}'' mythos, have time travelers turn into phantoms if they arrive somewhen they already exist. A memorable use of this concept occurs in ''ComicBook/WhateverHappenedToTheManOfTomorrow'', when a young Supergirl has traveled to the story's present (which is shortly after her death in CrisisOnInfiniteEarths), and casually asks her cousin why this ''hasn't'' happened.
-->'''Superman:''' ''(looking away)'' Right now Supergirl is... in the past.
* In most Silver-age Superman stories, if the Man of Steel travels to a time, past or future, where he exists, that other version gets tossed back to the "empty slot" he started from. This normally occurs in Superboy stories, so that Superman ends up wandering the Smallville of his youth.
* In the ''Final Crisis: Legion of 3 Worlds'' miniseries, Superboy-Prime punches his future self, causing both selves to dematerialize and disappear.
* In ''Final Crisis: Superman Beyond'' it was claimed that Superman and his MirrorUniverse counterpart, Ultraman, can't touch each other without causing an explosion that would destroy them both. Except in Limbo, where the rules are different. (Never mind that they've fought each other lots of times and that's never happened before.)
* Also, the first time one version of Supergirl met Power Girl, shaking her hand caused Power Girl to suddenly go berserk and attack everyone. This was attributed to them being alternate universe counterparts and it making reality glitch out for a moment or something, but it hasn't happened since.
** In the New 52 Supergirl issue #19, Supergirl and Power Girl meet up again with very different results, providing both a discussion/lampshading and subversion of this trope. It turns out Power Girl knew about the main DC Earth's version of Supergirl but didn't dare because she didn't want "the universe to explode if we touch." (What actually happens when they ''do'' touch is that [[spoiler: they exchange memories.]])
* In ''Timecop'', you can meet yourself and even talk to yourself, as long as you do not ''touch'', because "the same matter can't occupy the same space at the same time". When the BigBad does (with the hero's "help"), both selves melt and are erased from time.
* Major Victory of the MarvelUniverse, after he travels back to the present, has to be careful about this with his ''shield'', since it's CaptainAmerica's from the future; it's established that if the present and future shields ever make contact, Bad Things will happen. Oddly, this is not a problem when ''he'' meets his younger self.
* One ''{{Wolverine}}'' comic involves Jubilee going back in time. Her past self temporarily disappears.
* [[ContinuitySnarl One version]] of the death of ''ComicBook/JusticeSocietyOfAmerica'' villain Per Degaton (who was split into two versions -- one taking TheSlowPath, the other gaining access to a TimeMachine -- in 1947) has him being disintegrated when he and his "chronal duplicate" finally meet up in the 1980s.
* In one anniversary edition of ''ComicBook/SpiderMan'' involving DoctorStrange, Spidey was successfully able to see his past self (right before being bitten by the spider) and listen to advice from his future self on what seemed to be the end of his career (right before his future self is gunned down by the police for some unknown crime). Of course, seeing as the reason he was seeing this was because the fabric of reality was ''already'' in jeopardy because of Dormammu's plot to bring about the apocalypse, it's likely these two encounters couldn't have done any further damage.
* ''WesternAnimation/WinxClub'': The comic book version has a story where the Winx girls travel to the future. The scientist who invented the TimeMachine discourages them from meeting their future selves because of this trope.

[[folder:Fan Fic]]
* Around the time ''{{Titanic}}'' came out, a lot of ''Series/DoctorWho'' fanfics got written with the Doctor and co. visiting the doomed ship. One fanfic lampshades this by having the TARDIS appear on the ''Titanic'' only for the Doctor to moan, "Oh not again!" His companions look around and notice that a large percentage of the passengers are [[TheNthDoctor different versions of the Doctor]] and his various companions. "The TARDIS just seems to ''like'' the North Atlantic," he sighs.
* There was a ''WesternAnimation/MyLittlePonyFriendshipIsMagic'' fanfic which invelved Rainbow Dash travelling in time to set herself and Applejack up. Eventually, because of this, she, Applejack, and Twilight Sparkle are all jumping through time trying to make sure the right versions of themselves meet at the right time. It ends up with several dozen of each pony getting very confused. [[spoiler: And then [[FromBadToWorse Scootaloo gets caught up too...]]]]
** Another fanfic, ''FanFic/OnACrossAndArrow'', instead had dimensional travel replacing time travel. Twilight ended up sending the Mane Six into an alternate dimension where everyone is gender-swapped versions of themselves. She feared that if they met their alternate selves, they would explode or something. [[spoiler: Thankfully, this isn't the case in the slightest.]]
** Another variation shows up in the ''FanFic/PonyPOVSeries'', during the [[BadFuture Dark World]] arc when Twilight meets Minty Pie. The variation is that [[spoiler: both of them are reincarnations of G3 Minty [[note]]Twilight is Minty's "Light" (her soul) while Minty Pie is her "Shadow" (her physical appearance)[[/note]]]]. Due to this unusual variation, they're able to interact fine, without any problems, but when they make physical contact [[spoiler: they start to ''fuse together'']] which pretty much {{Mind Rape}}s them both. Fortunately, they break apart in time to avoid anything permanent, and while shaken, they're fine afterwards.

* ''Film/BackToTheFuture'', as noted on the quotes page. Probably the TropeCodifier in film, despite the trope being less a hard and fast rule and more of a general guideline.
** In the movies, nothing worse than fainting happens, and both Doc and Biff seem able to avoid even that by averting their eyes from themselves, or maybe just by being prepared for it. It seems that young Biff in the '50s had no basis to suspect that the "crazy old codger" was himself. It's also entirely possible that the "destroy the universe by temporal paradox" hypothesis was just that - a hypothesis, and there never was any real danger at all.\\
[[WordOfGod The two Bobs]] explain that Doc's concern is that the recognition of a past character meeting his or her future self could lead to an event that causes the paradox; for example, while the two Jennifers simply passed out when meeting each other, the producers explain that had "young" Jennifer fallen, hit her head and sustained a fatal injury she would not have had a future self to trigger the incident, resulting in a paradox.
** Likewise, in the [[VideoGame/BackToTheFuture 2011 video game]], the beginning of Episode 2 requires Marty to retrace his steps and keep his grandfather from getting killed while trying not to run into himself from a few hours ago (during the events of Episode 1). To make things even more confusing, the end of the final episode of the game has Marty arguing with Marty and Marty over which timeline is correct, while a confused Marty looks on. And yet Doc still says, while all of these Martys are having their little discussion, that them meeting each other could destroy the space-time continuum. Still seems fine at the moment.
** In LJN's ''Back To The Future II & III'' game for the NES, though, as you leave behind temporal clones of yourself every time you leave and then revisit one of the three time periods (1955, 1985, and 2015) in the ''Part II'' section, collision with your temporal copies causes you to lose a life.
** The ride at Universal Studios ends with a frantic order from Doc to leave the [=DeLorean=] before you encounter your past self coming in.
* In ''Happy Accidents'', Sam explains that it is impossible for time travelers to travel back within their own lifetimes; the only time travel possible is movement far into the past.
* In the film ''Film/SouthlandTales'', a huge part of the plot hinges on this twist, revealed late in the film:
** [[spoiler:Two time-travel-created copies of the same person shake hands with each other, setting off the end of the universe.]]
** Boxer Santaros avoids this [[spoiler:as someone has already taken the care to murder his double. Or perhaps the original.]]
* Subverted in ''Film/{{Primer}}'': when the protagonists start time traveling, they take elaborate precautions to avoid meeting themselves, but it becomes progressively more clear that (aside from creating permanent duplicates of themselves) these precautions are [[WrongGenreSavvy totally unnecessary]]. By the end of the movie, Aaron has drugged his past self's breakfast and stowed him in the attic, and is then promptly attacked by yet another, future, version of himself.
* Played with in ''Film/StarTrek'': [[spoiler: Spock Prime insists that Kirk cannot tell the younger Spock about his existence, with the heavy implication that some kind of universe-ending unpleasantness would ensue if he did. At the end, Spock Prime then seeks out and introduces himself to himself, and all-but-admits that he lied to Kirk, or at least deliberately left the universe-ending implication open. This was so that young Spock would eventually learn to develop the same bond that existed between Spock Prime and the Kirk of his reality.]]
* ''Film/SuperCapers'' seems to be fine with the two meeting each other, but if they physically touch each other... that's a different story.
* ''Film/DejaVu'' took care to avoid potential paradoxes like this in an unwritten way by ensuring that the Denzel who traveled back in time died without ever meeting his younger self.
* In ''Film/LostHighway'', Fred Madison buzzes himself on the intercom to tell himself, "Dick Laurent is dead." A DavidLynch film, of course, this is open to interpretation.
* ''Film/GodzillaVsKingGhidorah'' involves a Time Travel plot to remove the original dinosaur that became Godzilla from an island occupied by Japanese Troops in WWII. When one character asks why they couldn't bring one of the Japanese WWII veterans they knew with them, since they'd know where the dinosaur would be, the Futurians who had the time machine explain that if the same person was in the same point in time twice, it would cause a paradox that would cause one or both of the person in question to be wiped from existence.
* Played straight to gruesome effect in ''Film/TimeCop''. Physical contact with your other self leads to... well, it's [[BodyHorror not pleasant]]. As mentioned in the comic book entry, this is specifically due to the same ''matter'' occupying the same space; as long as they didn't touch, meeting was fine.

* In Creator/AnneMcCaffrey's ''Literature/DragonridersOfPern'' novels, time-travel is generally exhausting, but is substantially more so when traveling near oneself. This one's also an example in a StableTimeLoop world.
** Note that ''both'' selves will feel the effects. If you suddenly feel dizzy and weak for no reason, it might mean that future-you is in the vicinity.
** In ''Moreta: Dragonlady of Pern'', to stop a plague, Moreta and her dragon repeatedly loop back over the same time period. [[spoiler:Though her appearances are in many different locations, the repeated trips prove lethal.]]
** The most recent books imply that this problem is exclusive to dragonriders, and is the result of their telepathic bond with their dragon inadvertently becoming duplicated as a result of the time travel.
** This reaction is the reason Lessa survived Fax's attack on Ruatha Hold. When adult Lessa unwittingly traveled to the morning of the attack, child Lessa was awoken by the feeling of unease brought about by her adult self's presence and instinctively hid in the watch-wher's kennel.
* The Franchise/StarWarsExpandedUniverse novel ''[[Literature/TheThrawnTrilogy The Last Command]]'' by TimothyZahn has a variant of this trope, involving [[CloningBlues clones]] instead of time travel. Clones grown near others set up "resonance effects" in the Force, which can drive them insane, and when Luke encounters his clone, he finds the experience incredibly off-putting: there's a buzzing hum in the Force that makes it hard to concentrate or think, making it difficult to fight the clone. This only seems to affect clones whose growth has been accelerated to an extreme degree -- having them grow up in under a year, for instance -- and Force-sensitive clones, since the clone army in the prequels (who take about 10 years to grow up) doesn't have any problems with this. Thrawn figures out a way around it in the same novel, using lizards that can push away the Force to protect against the clone madness ''and'' dramatically decrease the time to grow them even further.
* One of the most important rules of TimeTravel in ''Literature/HarryPotter''? Don't be seen by yourself. You could panic and kill your past or future self. Notable for not really being a result of time travel so much as just being a duplicate. Though not spelled out explicitly in the books, this could have something to do with the existence of Polyjuice Potion -- in most non-time-travel related situations where you notice a duplicate of yourself, it means someone's up to no good. As Harry's own experience with TimeTravel, which was the only one the readers were able to follow, turned out to be a StableTimeLoop, the killing-your-past-self thing is unlikely to ever happen.
* The ''Literature/TimeScout'' series, which already presents time travel as potentially dangerous if you don't get the small details right, also avoids the meet-yourself problem by making it clear that you must never go back to a time when you already exist, either by having been born by then, or having previously visited via time travel; otherwise, since you can't be in two places at once, the "current" you who went back in time would simply wink out of existence the instant you arrived, and that's the end of you.
* In Creator/ConnieWillis's time-travel books, you simply ''can't'' go back to a time you've already visited. If you try it--or if your presence, for any reason, would cause a paradox of some sort--either the "net" (via which one time-travels) simply won't open, or it will deposit you slightly awry (in time and/or space) of your intended destination. Cosmic Censorship may be at work here: if you cause a paradox, then the next version of you is sent to a slightly different point (or the machine fails), repeat until there's no paradox.
* Subverted in ''Literature/TheHitchhikersGuideToTheGalaxy'' novel ''Literature/LifeTheUniverseAndEverything''; Ford Prefect and Arthur Dent arrive on Earth shortly before its destruction. Ford (who had, in the previous novel, explained to Arthur that history cannot be changed because [[StableTimeLoop it all fits together like a jigsaw]]), warns Arthur against phoning to warn himself. Not because it'll do anything to the timestream, but just because it won't work. It had already been noted in ''The Restaurant at the End of the Universe'' that no matter how many times you visit the restaurant in the title, which you would always do in the universe's last half-hour or so, you are guaranteed to never run into yourself "because of the embarrassment this usually causes," despite this being impossible. How the people responsible for the restaurant's operation pull this off is not explained, but it is [[LampshadeHanging lampshaded]] magnificently along with other things about Milliways by the Guide's repeated use of the phrase "This is, of course, impossible", and the restaurant's advertising slogan: "If you've done [[Literature/AliceInWonderland six impossible things]] today already, why not round it off with breakfast at Milliways, the Restaurant at the End of the Universe?".
* Creator/DianaWynneJones's novels often involve alternate universes and occasionally also time travel, which means they touch on this from time to time:
** ''Literature/ASuddenWildMagic'' has an example similar to the ''Stargate'' one below -- travel into alternate universes is possible, but causes instant death for anyone with a counterpart in that universe.
** In ''Literature/CharmedLife'', the character who manages to travel into an alternate universe does so via a method which cyclically displaces all her alternate selves, so every universe that had a version of her continues to do so. In fact, the main qualification for Literature/{{Chrestomanci}}, the enchanter who keeps all the other magic users in line, is that he have nine lives. This makes it possible for him to go to other universes comparatively easily: he has the extra lives because the alternate selves who would otherwise have them don't exist.
** But in ''Literature/ATaleOfTimeCity'', Vivian, Jonathan and Sam manage to be on the same railway platform in ''three different incarnations'' at the same moment and nothing happens - although they're careful not to be seen, it's just in case they change history. [[spoiler:More than they already have, anyway.]]
* In a SonicTheHedgehog novelisation ''Literature/SonicTheHedgehogInTheFourthDimension'', older and younger selves could co-exist, but not touch: if they did, they melted together somewhat painfully to become one self. Sonic did this ("Now I'm twice as cool"). He also did this with the evil [[MirrorUniverse anti-Sonic]], [[SdrawkcabName Cinos]], causing some alarm as to whether good or evil would win out in the resulting Sonic. [[spoiler: Good won.]] Imaginary creatures [[InTheirOwnImage threatening to rewrite history]] kept people from asking too many questions. Still, at the end, it was two positive Sonics plus one negative Cinos, which should equal one positive Sonic.
* In ''Literature/JohnnyAndTheBomb'' by Creator/TerryPratchett, the time-trolley sends Johnny forward in time just before his past self sees him, apparently to avoid paradox. It's still enough to shake the time travelling Johnny: "I saw the back of my own head! No-one's done that since the Spanish Inquisition!"
* In the ''VideoGame/{{Warcraft}}'' time-traveling trilogy ''WarOfTheAncients'', the character Krasus (a dragon shapeshifted into an elf) is unable to transform into his dragon form because he already exists, as his younger counterpart Korialstraz. Likewise, Korialstraz is unable to transform into anything else. The two are weaker the farther apart they are, but become stronger when they're close. The two work around this problem somewhat by giving each other one of their scales.
* Subverted in book six of the ''Literature/ArtemisFowl'' series, appropriately titled "The Time Paradox". The premise is that [[spoiler:in order to save his mother, Artemis must travel back in time to stop his past self from wiping out the last of the silky sifaka lemurs, whose brain fluid can cure her. Artemis meets himself, does battle with himself several times (and Butler, who's arguably the more dangerous of the two), and eventually the two Artemises collaborate to bring down Opal Koboi.]]
* In ''Literature/TheEndOfEternity'', there are time periods in regular time allocated for an Eternal to use, and naturally, you are not to use them twice. However, when the protagonist goes into regular time illegally, he doesn't keep track as well, so he ''almost'' meets himself (that is, he catches a glimpse of his past self). There don't seem to be any consequences, but he is extremely and irrationally terrified.
* In ''Literature/TheBookOfTheNewSun'', Severian feels absolutely certain that if he accidentally met himself while time traveling, one or both of them would go mad and kill the other.
* Hinted to normally be the case for the Eternal Champion in Creator/MichaelMoorcock's related stories. Different incarnations do meet each other and even team up from time to time under suitably unusual circumstances, but once the emergency allowing for the encounter passes they inevitably have to part ways again shortly thereafter (sometimes downright abruptly) to avoid putting too much unspecified strain on the multiverse.
* ''Literature/TheRifter'': Averted. When Kahlil arrives in a Basawar where history was changed, the version of himself in the new time is already dead. He does have two sets of memories coexisting confusingly in his head, though. Eventually, he finds the bones of his other self and they merge into his body; at the same time, he feels like his two histories have become truly integrated into one personality.
* In one of the ''Literature/RIsForRocket'' stories, a man participating in a time-travel safari has this explained to him. The 'bump' they felt just before arrival was the time machine leaving at the end of the safari, the tour guide explains that "Nature doesn't allow that kind of thing- man meeting himself".
* In Larry Maddock's "Agent From T.E.R.R.A." series about time traveling cops trying to defeat a rogue time traveling agent turned conqueror, this is explicit. Why is never explained, but at least once per book the plot hinges on one or more people being prohibited from entering a certain period of time (minutes to hours) because they were then (sic) already. One very nice feature of this series is that several times the main character will race toward his time machine, immediately phase out of the time stream, and then relax: going to bed, taking care of wounds, preparing and eating a hearty meal, etc. He reflects that he could spend literally years preparing to go back to the precise micro-second he left, so now there is no hurry at all.
* This rule is on the rather extensive list of time travel restrictions in Creator/DeanKoontz's ''Lightning''. [[spoiler:This becomes a serious problem at the end of the book when someone travels very, very briefly to a point in the future and can't return to it, even though he really, really needs to.]]

[[folder:Live Action TV]]
* This has, not surprisingly, come up a few times on ''Series/DoctorWho'', though he has had no trouble (usually) meeting past versions of himself.
** The Doctor, while visiting a fascist MirrorUniverse of Earth, claims that if that universe's evil version of the Brigadier were to meet the original it would destroy both universes. [[spoiler:The evil Brigadier wants to force the Doctor to save him from his Earth, which volcanic eruptions would destroy in a matter of minutes.]]
** In "Father's Day", traveling to the same place twice allows Rose to create a TemporalParadox; later in the same episode, she gives more power to that story's ClockRoaches [[spoiler:by touching her own infant self]]. It's revealed that cleaning up paradoxes without such things happening is part of what the Time Lords did, but now that (for the most part) there ''aren't'' any Time Lords any more, there's no one to prevent such things from happening. This may explain why no flying killer time monkeys appeared during any serial with a title of the form "The [number] Doctors."
*** The reason that it was such a dangerous deal when Rose touched her infant self is because she had accidentally caused the time-sterilising monsters to show up by [[spoiler: saving her father's life when he had originally died]]. The later paradox of touching her infant self is only dangerous because it lets the monsters into the church. It's implied that in normal circumstances, this would just be a paradox and nothing dangerous would necessarily come of it.
** ''Mawdryn Undead'' shows that when the Brigadier met his younger self, the resulting "Blinovitch Limitation Effect" had the effect [[spoiler:of giving the younger Brigadier a nervous breakdown, as well as enough PureEnergy to kill a half-dozen previously immortal aliens]]. In terms of the Brigadier, this created a neat StableTimeLoop.
** Of course, the original series also has several stories where [[TheNthDoctor multiple incarnations of the Doctor]] were brought together in the same place at the same time, without any apparent ill effects to the TimeyWimeyBall that is the {{Whoniverse}}. (Most of the time, this could be explained by the fact that the multiple incarnations aren't ''entirely'' the same person. But a couple of times two of the same incarnation have met.)
** The 2007 special ''Time Crash'' blamed Creator/PeterDavison's Doctor aging 30 years on temporal weirdness resulting from being in the same room with himself. (This same explanation may be used to justify the relatively minor differences in appearance occurring in multi-Doctor stories in the original series- the biggest of which (besides TheOtherDarrin example of the new First Doctor) was the obviously-older Second Doctor in "The Two Doctors". But then the Franchise/DoctorWhoExpandedUniverse explains the latter, anyway.)
** Funnily enough, this flies out the window in "The Big Bang" when Amy meets her seven-year-old self ("Amelia"), and touches her a couple of times in sizing her up. Possible FanWank[=s=] include a) Amelia is from an alternate timeline, so they're not technically the same person (apart from anything else, she [[spoiler:vanishes minutes later, so she obviously doesn't grow up to become Amy]]), and b) with the whole Universe toast, save for the Earth and [[spoiler:the TARDIS explosion acting as a substitute Sun]], there are probably no ClockRoaches, and really more pressing matters at hand than a couple of silly old paradoxes.
** Lampshaded later when Amy mentions that she met herself, and Rory responds that, to be fair, [[spoiler:the universe did blow up]].
** However, another time, two Amys (Amies?) met and the problem was being able to save only one. There's also a comic relief special where Amy meets herself from a few minutes down the line. The only danger is to Rory ([[GirlOnGirlIsHot risk of hormone-based overload from watching Amy flirt with herself]].)
** Another time, the "limitation effect" was a ''small spark'' between two of the same Sonic Screwdriver.
** In BigFinishDoctorWho story "Time of the Daleks" the Eighth Doctor claims if two versions of someone meet the older version will be destroyed, as the younger version needs to become the older version. [[spoiler:It is revealed this is the reason General Learman destroyed the Dalek Pilot. It was because of the lack of a pilot that the Daleks turned her into one and they are trapped in a StableTimeLoop.]]
** "[[Recap/DoctorWho50thASTheDayOfTheDoctor The Day of the Doctor]]" episode starts with three incarnations of the Doctor meeting up thanks to [[spoiler:the Moment]] opening rips in space-time. The Tenth and Eleventh Doctors meet the War Doctor, a previously-unmentioned incarnation that existed between the Eighth and Ninth Doctors who renounced his name during the Time War. [[spoiler:Taken UpToEleven at the end, when ''all'' thirteen incarnations meet at a single point in time to put Gallifrey into another dimension, including the as-yet-nonexistent Twelfth Doctor]]. However, it's specifically mentioned that only the Eleventh Doctor ([[spoiler:and the Twelfth]]) would remember this happening, as the events are asynchronous with the timelines of the earlier Doctors.
** In "[[Recap/DoctorWhoS34E4Listen Listen]]", the Doctor tries to get Clara to travel to her childhood, but Clara (who is at the time psychically linked with the TARDIS) gets distracted and travels to her LoveInterest's childhood. The Doctor doesn't yet know that and tells Clara to stay in the TARDIS in order to avoid meeting her past self. Later, [[spoiler:Clara finds out she accidentally traveled to the Doctor's own childhood (as the First Doctor) and created a StableTimeLoop. She demands that the Doctor leave this time and not try to find out where and when they ended up]].
* On another {{Whoniverse}} series, ''Series/{{Torchwood}}'':
** The never-aging Captain Jack, [[spoiler:while trapped in 1901, orders himself [[HumanPopsicle cryogenically frozen]] to avoid meeting in the intervening decades before the present,]] and to prevent himself from meeting his [[Recap/DoctorWhoS27E9TheEmptyChild "The Empty Child"]] self in 1941 and [[Series/DoctorWho "Boom Town"]] self in 2006, as well as the version of him that has lived through all this already.
** At the end of "Kiss Kiss Bang Bang," the whole team, having gone back to earlier that day, are told that they must avoid themselves.
** It seems, though, that they're more worried about changing their own history than a TimeCrash resulting from simply being in the same place, since the same person being in the same place has happened so many times safely. If you interfere where you've ''already'' interfered, though, the ClockRoaches will getcha, as seen in the aforementioned DW episode, Father's Day.
* In ''Series/SevenDays'', whenever Parker travels back in time, his younger self, the sphere, and one trip's worth of their alien fuel source vanishes.
* Generally averted in ''Series/{{Heroes}}'', as several characters have met past or future selves without anything happening because of it. It's occasionally [[DiscussedTrope discussed]], though:
** The GenreSavvy Time Traveler Hiro Nakamura runs into his future self, only to cry in dismay, "Aren't you afraid of disrupting the Space-Time Continuum?"
** And before that, when he realizes he's accidentally ended up talking to his ''past'' self on the phone, he hangs up instantly, exclaiming "[[Film/BackToTheFuture Great Scott!]]"
** Season four had Samuel explain to Hiro the dangers of the [[ButterflyOfDoom butterfly effect]] when Hiro goes to change an event with major influences on events surrounding his past self.
* A non-time-travel variant occurs in ''Series/StargateSG1'': anyone who travels to another dimension is subject to quantum seizures if they have a living counterpart. You can survive for short periods, and are exempt if your counterpart is dead, which saves Daniel in one episode, Kowalsky in a second, and Carter in a ''third''. Time travel, on the other hand, doesn't seem to have this effect in any way.
* This trope gets smashed into the ground with the character of Martin Summers in ''AshesToAshes''. Then again, in the light of the finale, [[spoiler: whether it ''actually'' applies is debatable]].
* In ''Crime Traveller'' this is one of the rules of time travel. It was never entirely clear what would happen if someone met their past self, but the implication was that it wouldn't be good. Strangely, one of the other rules was that the time travellers needed to be back in the room with the time machine at the moment they left. In theory, this means that they should see themselves leaving and break the first rule but that was never shown to happen.
* Creator/{{NBC}}'s ''{{Journeyman}}'' doesn't even bother to explain why you shouldn't be seen by your other self -- it just assumes you already know that. Though the main character is able to have a fistfight with his past self, so it's at least OK as long as your past self doesn't get a good look at you.
** Like ''Film/BackToTheFuture'', this winds up less of a cosmic rule and more of a guildeline to prevent paradoxes, since the timeline is ''very'' malleable (case in point, the time he [[spoiler:left a digital camera in the 1980s, which subsequently retconned his own son out of existance]]). But the protagonist is very GenreSavvy from the get-go.
* ''Series/RedDwarf'' does this too, at the end of series 6. The crew's future selves travel back in time to meet them as they are at that point for some repairs. Kryten admonishes the crew to not contact the future Dwarfers to avoid gaining inappropriate knowledge of the future. They watch instead, and despise what they became. The result? 'I say we fight. ... Better dead than smeg.' From the snivelling, whining coward (according to everyone, including himself) Rimmer. Causes a paradox anyway, but that's what they ''wanted'' at that point.
* In the outtake "Orchid Orientation Film" from the ''Series/{{Lost}}'' season 3 [=DVDs=], a time travel experiment involving bunnies apparently goes wrong. The scientist shields one of two identical bunnies from the other and shouts, "Don't let them near each other!" The series proper averts the trope, though.
* The ''Series/{{Star Trek|The Original Series}}'' episode "The Alternative Factor" involves two identical men named Lazarus. One is from our universe and insane. The other is from an anti-matter universe. Apparently, if both meet in the same universe, it destroys both of them. No explanation was given why the anti-matter Lazarus didn't blow up upon contact with any matter from our universe. The episode ends in a FridgeLogic moment where Kirk traps the two between universes, so they can fight it out for eternity. Apparently no one thought of killing or imprisoning the insane one.
* Played with in the ''Series/SoWeird'' episode "Pen Pal". Annie is able to meet herself from a parallel universe without consequence, but only one of them is supposed to exist in that dimension, so they're unstable. When they touch each other, Parallel Annie is destroyed.
* Averted in ''Series/KamenRiderDenO''. TheLancer Yuuto is the past version of modern character, who recruited his younger self personally. In TheMovie, Ryotaro meets his ten-year-old self, the shock causing them to faint and giving the present version EasyAmnesia; this creates a problem since it means his [[NobleDemon Imagin partners]] can't [[SharingABody "borrow" his body]] to become Den-O until his memory is restored.
** TheMovie utterly destroys this trope by having Yuuto grab three versions of Ryotaro out of the normal time stream, hitting them with a powerful sedative, and then having three of his four Imagin possess the unconscious Ryotaros, allowing all four primary versions of Den-O to exist at the same time and participate in the final battle.
* ''Series/KamenRiderFourze'' destroys this trope even so more than Den-O in the CrossOver film ''Movie Wars Ultimatum''. When the five-years-later version of Gentaro Kisaragi comes back in time to our present, the '''very first''' thing he does is pull his past self aside (via tackle), say "Hi, me!", and ask to borrow his TransformationTrinket [[spoiler:since he destroyed the future version as a SecretTestOfCharacter for one of his students]]. Present-Gentaro's first reaction is of course utter shock, but after he calms down he gladly loans the belt.
* In ''Series/{{Sanctuary}}'', after [[spoiler:Helen travels back in time to stop Adam from destroying Praxis, she is stuck there and has to take TheSlowPath, while remaining in hiding from anyone who might know her, including her other self]]. It's not stated that physical contact would be disastrous, but [[spoiler:Helen doesn't recall meeting another Helen, so she can't risk being seen]].

[[folder:Tabletop Games]]
* In the core ''TabletopGame/{{Timemaster}}'' rules (from the defunct Pacesetter Games), it was impossible to meet up with yourself. Any attempt to travel to a time period you already occupied put you into a "Loop Trap" -- you'd basically be "stuck" reliving the time covered by the loop over and over. Of course, you wouldn't realize this, because it would be the first time through every time. Better hope one of your teammates is willing to pull you out of the loop. ''Timetricks'', a Timemaster supplement meant for more experienced groups, included a little gadget that would let you bypass a Loop Trap for a short period of time, assuming it worked.
* Played with in ''TabletopGame/GeniusTheTransgression''. Interacting with your younger self is relatively safe compared to all the other stupid things you can do during a time-travel jaunt. However, going back in time to the same moment twice and coming into contact with your own time-traveling self is an excellent way to drive yourself all sorts of crazy -- when the game describes an act as "about the stupidest thing you can do without a death ray and a bottle of tequila," you ''know'' it's a bad idea. It also [[GameplayAndStoryIntegration handily prevents]] an [[MesACrowd "army of temporal duplicates"]] scenario from coming to pass.
* In the obscure ''DungeonsAndDragons'' second edition {{Splat}}book ''Chronomancer'', this is in effect for anyone who travels to a time period even close to when they already exist (either from their natural life or other time travel jaunts). Various forms of ClockRoaches will attempt to force a time traveler into a temporal frame they didn't previously exist in, some rather destructive. Eventually, should they avoid all those, a TimeyWimeyBall will automatically force the offending time traveler to a point forward where it's no longer an issue.
* ''''TabletopGame/{{Continuum}}'' not only [[InvertedTrope inverts this]], but expects this will happen and has rules (both etiquette and mechanical) on how to handle such an event (dubbed a "Gemini incident"). Remember to respect your elders.
* ''MagicTheGathering'' has rules in place to partially enforce the alternate-universes version of this, although whether the time-travel version is also enforced depends on whether you're a legendary creature or a planeswalker:
** The "legend rule" applies to legendary creatures (or legendary permanents of any type). Legendary permanents represent unique, named characters, places, or objects rather than generic ones; the legend rule states that if a legendary permanent entering the battlefield shares a name with one already on the battlefield, both are sent to the graveyard. However, two legendary permanent cards could represent the same character without sharing a name. For example, [[http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=106398 Kamahl, Pit Fighter]] and [[http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=220490 Kamahl, Fist of Krosa]] represent the same character but have different names, so they could coexist on the battlefield.
** The planeswalker card type has a similar rule to the legend rule, only it refers to a planeswalker's subtype; if a planeswalker entering the battlefield shares a subtype with one already on the battlefield, both are sent to the graveyard. This is because there are no generic planeswalker cards; each represents a specific character, and the subtype indicates which character is represented. This means that two planeswalker cards that represent the same character without being the same card still destroy each other. For example, a popular strategy to combat [[ObviousRulePatch one-time]] GameBreaker [[http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=195297 Jace, the Mind Sculptor]] was to either remove him by casting his less-expensive version [[http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=205960 Jace Beleren]], or use the fact that Jace Beleren is less expensive to put him into play before a Mind Sculptor is cast at all, meaning the other player would have to waste a Mind Sculptor card in order to clear the way for another one. Since both represent the same character (in game terms, since both are Planeswalker - Jace), they destroy each other by being in play at the same time.

[[folder:Video Games]]
* This happens intermittently in ''{{Terranigma}}''. [[spoiler:When the 'Dark World' Ark meets the 'Light World' one's spirit, Ark is instantly killed and reincarnated as a baby with a mixture of the Light's spirit and the Dark's memories and personality. Later, his girlfriend Elle meets the Light World version of herself... and nothing happens. In fact, for a second it seems they're about to get into a CatFight.]]
* In ''ShadowOfDestiny'', multiple versions of the same person can be in the same room and even talk to each other without problem. Physically touching your past or future self, however, results in both of you ceasing to exist. [[spoiler:This becomes a plot point in various possible endings.]]
* In the small InteractiveFiction game ''All Things Devours'', sighting yourself - or causing your earlier self to notice anything you didn't - causes the space-time continuum to go blert with the force of a nuke. This is a problem, as the main character is trying to destroy her prototypical time machine with a range of minutes, inside a military base that nobody could infiltrate alone. Fascinating choreography, shameless TrialAndErrorGameplay.
* In ''LegacyOfKain'' the moment when two incarnations of the Soul Reaver meet a paradox is created, and the resulting distortion of the timestream allows to make changes in history, which is usually written in stone, this happened four times in the games.
* In ''VisualNovel/FateStayNight'', it's explained that having copies of the same person present in the same time period causes a strain on reality that will manifest on both copies, as reality cannot truly distinguish between them. They will slowly start siphoning knowledge and skills from each other merely by being around each other, and eventually, one or the other has to go. This is compounded more directly in-story by [[spoiler:the future copy trying to kill his past self in the hopes a TemporalParadox will erase him from existence]].
* Subverted in one of the ''SamAndMax'' episodes, involving a lot of time travel. The duo meet their past selves from a year and a half earlier, putting them early in the previous season. It doesn't mess up the universe, but it does result in Sam and Max [[spoiler:getting trapped in the past and having to relive the entire year and a half over again because the past versions of themselves steal the time machine]].
* Through the mixed-up and convoluted story of ''VideoGame/SonicTheHedgehog2006'', this essentially takes place. There probably isn't a time where there aren't at least two Sonics/Silvers/Shadows running around at the same time, just in different locations. For example, late in the game, Sonic, Silver, and (strangely) [[VideoGame/SonicRushSeries Blaze]] find themselves on a cliff, while Doctor Eggman's Egg Carrier crashes into the side of a mountain, leaving Sonic to believe Elise is dead. Silver then suggests Sonic goes back in time to rescue her. While this happening, [[spoiler:Sonic has already done so. He and Elise had already escaped the crash just as the carrier exploded.]]
** Averted in ''VideoGame/SonicGenerations''. The entire gimmick of the game revolves around both Classic and Modern Sonics (and they meet up rather quickly.) In addition, there is also a meeting up of Classic and Modern Tails [[spoiler: and Classic and Modern Eggman - who both pilot the final boss.]]
* ''VideoGame/{{Portal 2}}'' has the quote at the top of this page.
* In ''{{Onimusha}} 3: Demon Siege'', [[spoiler:player-Samanosuke comes back in time and sees his alternate-timeline self laying slain by Nobunaga. Touching his Oni Gauntlet merges the two together and the combined power from the paradox enables Nobunaga's defeat.]]
* {{Inverted}} in ''VideoGame/KingdomHeartsDreamDropDistance''. There, you ''can't'' travel through time unless you're already at your destination [[spoiler:or you abandon your body.]]
* Used as a plot point in ''VideoGame/SuperRobotWarsZ'' 2, albeit with dimensional travel rather than time. Under the physical laws of the multiverse, a person cannot travel to any parallel world where a version of themselves is already present; when the heroes from ''SRWZ 1'' travel to the new world, the [[MazingerZ Mazinger]] [[GreatMazinger Team]] and the [[GetterRobo Getter Team]] get left behind since the ''Z 2'' world already has the ''Anime/ShinMazinger'' and ''Getter Robo Armageddon'' versions of them. The rest of the heroes didn't realize this until they met the parallel Mazinger and Getter Teams and started asking questions about their allies, which naturally they couldn't answer.
** Also used with a twist in regards to the OriginalGeneration. In the ''VideoGame/SuperRobotWarsAlpha'' timeline, Ingram Plisken creates an incredibly powerful robot called the Astranagant; however, in the ''VideoGame/SuperRobotWarsOriginalGeneration'' timeline, he instead upgrades his old R-Gun into the R-Gun Rivale, which is close to but not quite the same thing. The official reason given is that the Astranagant (and its pilot) are the [[GuardianOfTheMultiverse Guardians of the Multiverse]], so there can only ever be ''one'' Astranagant in all of existence, and not-quite-knockoffs like the R-Gun Rivale are the closest any other universe can get.
* A non-paradox version of this in ''VideoGame/FireEmblemAwakening'' is the stated reason the [[KidFromTheFuture Kids From The Future]] leave once their present selves are born. The future children don't want to live with their present selves because it would negatively impact their childhoods. Averted for [[spoiler:Lucina]] and [[spoiler:Noire]], as the former is already born while the latter stays with her present self to [[SetRightWhatOnceWentWrong right her relationship with her mother]].
* Some levels in ''The Adventures of Shuggy'' generate clones of Shuggy at set intervals. If he makes contact with any of his clones, he dies and has to start the level over from the beginning.
* Surprisingly averted in ''VideoGame/{{Blazblue}}'', which most of its plot consists of time travelling. [[AnimatedArmor Hakumen]], comes from an alternate timeline which was similar to the canon one the series takes place in, save for a few major details changed. He makes his way to the canon storyline by being flung back in time 100 years prior to the beginning of the first game and becoming one of the world's legendary heroes. His story path in the first game reveals he [[spoiler: is the reincarnation of the series` deuteragonist, Jin Kisaragi]]. The two have encountered each other quite a few times throughout the series, but there doesn't seem to be any adverse affects of this trope happening.

* Example in ''Alfheim'': [[http://www.bookelves.com/archive/alfheim-current.php?p=54 "One false move and we're all pink wombats!"]]
* Mostly averted in ''Webcomic/SchlockMercenary'', except that Schlock's BizarreAlienBiology causes him to involuntarily merge with his doppelganger.
* Happens in ''Webcomic/SluggyFreelance'', though for far more mundane reasons.
-->"If some guy came up to me claiming to be my future self, I'd sense a setup and toss him overboard, figuring if he ''was'' me I would have expected that."
** Meeting and hanging around with yourself in Timeless Space is generally considered a bad idea, since it means you've managed to escape once before (and returned later), and most denizens of the place would ''kill'' to find out how you did it (or to prevent their enemies from finding it out). Having a normal twin is even more dangerous, since people will just think you managed to escape, even though you never did.
* One of the results of [[http://www.penny-arcade.com/comic/2004/5/3/ Gabe using a time helmet]] in ''PennyArcade'' and meeting himself apparently involves reality breaking into warring shards floating in a sea of un-time.
* One of the big issues in the first time travel arc of ''Webcomic/CaseyAndAndy'', when several of the main characters end up back in the time when they were in high school. But it's easy to find the young Casey and Andy, since you only have to [[spoiler: look for the biggest explosion.]]
* Played straight (so far) in ''SailorSun'' as Honey (the KidFromTheFuture) seems to be unaware that her present self (Brady) was sent to live with her aunt shortly before Honey arrived in our time and it's implied that her mother may be deliberately keeping her in the dark.
* In {{Persona 3 FTW}}, the {{Persona 3}} Protagonist meeting his DistaffCounterpart (and making out with her) causes the Earth to implode.
* In ''CaptainSNES'', this appears in [[http://www.captainsnes.com/2010/12/31/7-effect-and-cause/ this side comic]], and is explained on the next page.
* ''Webcomic/{{Homestuck}}'' has no ''explicit'' taboo against multiple selves meeting each other, as long as their interactions form a StableTimeLoop. However, selves from splinter timelines are "doomed" and will inevitably be culled from existence: if you go back in time to prevent your past self from triggering the BadFuture you came from, don't expect to live long afterward.
* In ''{{Frankie and Stein}}'', Shelly claims that if they meet themselves in the future, their [[YourHeadASplode heads will explode]]. Later, [[spoiler: she and Stein meet their future selves, and future Shelly disproves this theory and tells them to calm down, subverting this trope quite nicely.]]
* One storyline in ''DragonTails'' involved the dragons travelling through time. At one point, they end up back in their forest a few years earlier, on Christmas day. Cornelius and Abijar want to go join their past selves to celebrate Christmas, at which point Enigma brings up this trope. [[http://dragon-tails.com/comics/archive.php?date=010704 It does not turn out as he planned]].
* The old webcomic ''Jamie and Nick'' featured the alternate dimension version. Traveling to an alternate dimension is always a bad idea, even if your counterpart is already dead. If your counterpart is alive, the universe will eventually notice and erase both of you from existence. If your counterpart is dead, the universe gets confused and gradually rewrites your memories with the memories of your other self.

[[folder:Web Original]]
* In TheRandomverse, [[spoiler: Lex Luthor]] explains away a potential plot hole by explaining this trope. Lampshaded by [[MediumAwareness Deadpool]], of course.

[[folder:Western Animation]]
* The ''WesternAnimation/Ben10AlienForce'' episode "Paradox" has Kevin's car turned into a literal rust bucket by a time monster that ages anything it touches. At the end of the episode, Paradox gives them a new (old?) car, with the warning that it will "explode like antimatter" if it comes into contact with anything else from 1976. However, earlier in the episode, Paradox spoke to an alternate universe version of himself without any ill effects - granted, they did not shake hands or anything, but still... [[note]]WordOfGod is that Paradox was joking about Kevin's car.[[/note]]
** Other instances of time travel are often explained away as being alternate realities. WesternAnimation/{{Ben 10}} Vanilla featured a future Gwen bringing child Ben and Gwen back to her time to convince [[FutureBadass Ben's future self]] [[HelpYourSelfInTheFuture to slow down with his hero gig]]. He clearly didn't recall the encounter. Later, in UltimateAlien, another version of [[MyFutureSelfAndMe Ben from the future appears]], remembering the previous time travel adventure, but not of meeting himself as a teenager. One could argue the very act of meeting [[AlternateSelf another version of yourself]] causes [[ForWantOfANail your own future to change]] (while leaving his present/past intact, as for him, it already happened).
* One episode of ''BuzzLightyearOfStarCommand'' has the heroes encounter their alternates from another dimension (where the biggest difference was that Buzz himself was evil). XR warns the team to avoid touching their counterparts -- just as the two Boosters greet each other with a hug. When nothing happens, XR remarks on the trope not applying.
* In the ''WesternAnimation/{{Futurama}}'' movie ''Bender's Big Score'', we learn that duplicates of persons or objects created by time travel are automatically doomed, as a form of EquivalentExchange. This becomes a plot point as we learn that [[spoiler:Leela's new fiancee is another version of Fry, and calls off the wedding rather than subject Leela to the grief caused by his ultimate demise. And sure enough, he dies at the end in a HeroicSacrifice.]] At the very end, hundreds of Bender duplicates appear and [[spoiler:as they explode one by one, the sheer volume of temporal anomalies leads to a rupture in the universe - and a SequelHook for the next movie.]] This is ''almost'' a subversion as until time corrects the paradox by destroying the duplicate duplicates are free to touch and interact with each other.
-->'''Scammer Alien:''' I met my past self in a bar for a drink, one thing led to another, and [[ScrewYourself we ended up back at my place]]. Or should I say, "our" place. (passionately embraces duplicate)\\
'''Everyone Else:''' Ewwwww!!!\\
'''Scammer Alien:''' Oh come on, you prudes!.
* In the season finale of ''WesternAnimation/{{Superjail}}'', the Warden goes back in time to his own trial to save himself, and by the simple act of hugging his past self, causes the very fabric of reality to completely and utterly fall apart.
--> ''"Stop touching yourself!"''
** Cue RocksFallEveryoneDies - which is [[DeathIsCheap pretty much the standard]] for any given episode.
* On ''ThePenguinsOfMadagascar'', Kowalski invents a time machine, but a future Kowalski has come to get Private to stop him, warning him that if both Kowalskis were to ever meet, it would cause a rip in the space-time continuum. Also, a ''third'' Kowalski has come to get Skipper to keep the time machine from being destroyed. When the two future Kowaslkis meet, they reassure the others that it's okay, as long as the ''original'' Kowalski doesn't see them. And that's when original Kowalski sees them, causing the space-time rupture that led them all here in the first place.
* In ''TazMania'', Marvin the Martian's team-up with Taz. For whatever reason, hooking Taz up to some big gyroscope thing allows him to spin them through time. Marvin is always cautioning him not to meet himself, as "It results in an EarthShatteringKaboom." Of course, he forgets this advice. The result was somewhat similar to the ending of that ''TinyToonAdventures'' episode where Buster, Hamton and Plucky drank beer.
* Another ''WesternAnimation/LooneyTunes'' short has a humorous non-time travel invocation of the trope. Playing hide and seek with Prissy's little "egghead" son, Foghorn Leghorn hides in a storage bin. The young genius plays with a slide rule, scribbles down some calculations, marks an "X" on the ground, and proceeds to extract Foggy from that very spot with a shovel. Foghorn protests, but the little Einstein just points to his calculations. As the scene ends, Foggy passes the bin in which he had hidden, and starts to curiously lift the lid, but stops himself.
--> '''Foghorn:''' No, I'd better not look. I just ''might'' be in there!
* In the DCAU, specifically in ''WesternAnimation/JusticeLeague'', this is the explanation used by Vandal Savage for why he doesn't go to the past and stop himself from destroying the world; considering that Vandal Savage is an immortal caveman who has lived out virtually all of human history, that means he effectively can't time travel at all. He specifically says that it's a result of his particular time machine's design rather than an immutable law of the universe. In fact, Batman travels to the future and meets the ''WesternAnimation/BatmanBeyond'' version of himself in a later episode, with no consequences. Admittedly, time outright ''collapsed'' in that episode, so "no consequences" is a bit of a relative term. In the short term, at least, it turned out fine.
** Other than the alterations Vandal wanted to cause, his meeting of his past self didn't seem to cause any major damage.
** The ''WesternAnimation/BatmanBeyond'' version of Batman had no recollection of his younger self's perspective of their meeting or the events that led to the meeting. That said, the timestream was ''already'' pretty much in flux by that point for unrelated reasons.
** Static traveled to ''Batman Beyond'' and briefly met his future self. No EarthShatteringKaboom, and his future self didn't seem the least bit surprised.
* ''BumpInTheNight'': Mr. Bumpy unwittingly insulted Squishington, who locked himself away as a result of it. Bumpy then went back in time to prevent that but failed. Then he went back ''again'' and failed ''again''. The three Bumpys then got an advice from Future Squishington and apologized to their friend. When Squishington asked the Bumpys why there were three of them, they disintegrated because they couldn't agree on which ones had to leave.
* In the ''FantasticFourWorldsGreatestHeroes'' episode "Recap/OutOfTime", HERBIE frantically reminds Susan, Ben, and Johnny of this rule, alleging that "the universe could implode!... or something" if it's not followed.
* ''WesternAnimation/{{Gargoyles}}'': When David Xanatos went back in time to [[StableTimeLoop set the events that would lead to his past self receiving the ancient coin he sold in order to have the money to]] [[SelfMadeMan start his fortune]], Demona went back as well and tried to persuade her past self into a FaceHeelTurn earlier than she did in the original timeline. She failed but claimed she remembered that meeting from her past self's point of view and stated the past couldn't be changed.
* ''PinkyAndTheBrain'' received a visit from their future selves with a plan to go to the future (a future ruled by cockroaches) to steal a kit to take over the world. [[FailureIsTheOnlyOption Not only did the plan fail, as usual]], but they ended with several Pinkys and several The Brains.
* ''WesternAnimation/KimPossible'': In "A Sitch in Time", when Kim defeated Drakken, Killigan and Monkey Fist, Shego got a visit from her future self, who advised her to steal the time monkey and use it to take over the world. Past Shego first assumed Drakken cloned her.
* ''WesternAnimation/LiloAndStitchTheSeries'': Jumba invented a time machine and installed a paradox inhibitor to prevent its users from meeting past selves. The users would basically rewind time. Lilo and Stitch once forgot to use the inhibitor and met their past selves but no disaster seems to have happened from it.
* ''TheFairlyOddparents'' had three episodes of this instance:
** "Father Time" had Timmy and his fairies travel back to the 1970's. Timmy found out a young Bill Gates was Cosmo and Wanda's godchild. Cosmo and Wanda met their past selves but nothing bad happened from it. When he was back in present time, a future Timmy showed up with ''his'' Cosmo and Wanda, destroyed the remains of a trophy and told present Timmy he'd thank him for it. [[NoodleIncident One can only wonder why]].
** "The Secret Origin of Denzel Crocker" had Timmy and his fairies going back to 1972 and Timmy found out Cosmo and Wanda were Crocker's fairy godparents back then. It's unknown if past Cosmo and Wanda recognized their present selves, but past Jorgen recognized present Jorgen.
** "Channel Chasers": Coming from a BadFuture, an adult Timmy went back to his childhood to prevent Vicky from taking over the world.
* In ''WesternAnimation/TheSmurfs'' episode "No Time For Smurfs", Handy accidentally creates temporal copies of himself, Brainy, Clumsy, and Smurfette when they mess around with Father Time's Sands Of Time hourglass by first causing time to go backwards and then causing time to repeat previous events such as their discovery of Father Time's workshop. Averted in that nothing disastrous happens, but the temporal copies do inform Papa Smurf that they have seen what they think are copies of themselves in a cave. Fortunately, by the end of the episode, Father Time and Mother Nature straighten the whole mess out, and the temporal copies vanish.
* In an episode of ''WesternAnimation/{{Chowder}}'', Mung Daal and Chowder travel back in time to [[SetRightWhatOnceWentWrong correct a cooking mistake Mung made as an apprentice]], where Mung insists that they ''not'' interact with his past self to avoid space-time from unraveling. Naturally, Chowder doesn't listen and brings Mung's past self back to the future with them, wiping out their entire universe ([[StatusQuoIsGod not that it sticks]]).
* This Trope is ''Defied'' in the ''WesternAnimation/DextersLaboratory'' movie ''Ego Trip'', where the Dexter everyone's familiar with uses his time machine to form a team consisting of himself and his Teenage, Adult, and Elderly selves to SetRightWhatOnceWentWrong, courtesy of Mandark's tampering. There was no EarthShatteringKaboom, ''despite'' the fact that [[spoiler:Mandark used the same trick to fight them. In fact, after Dee Dee intervenes, she accidentally crates a future utopia.]]
* In the 2014 WesternAnimation/MrPeabodyAndSherman movie, going back to meet yourself is technically okay, but coming into contact with your past/future self causes the two to combine into one and the timeline to dissolve, dragging various historical figures and landmarks into the present. When Mr. Peabody realizes Sherman is at risk of causing this paradox, he runs in and shouts "Sherman, I have to stop you from [[DoesThisRemindYouOfAnything touching]] [[ADateWithRosiePalms yourself!]]" which doesn't bode well for his social services evaluation.
* [[spoiler:[[AvertedTrope Averted]] in Disney/MeetTheRobinsons at the end where Lewis meets his future self Cornelius Robinson, they even touch and nothing bad ever happens. The same applies when Goob unknowingly encounter his future self, The Bowler Hat Guy.]]
* In the ''WesternAnimation/CodenameKidsNextDoor'' episode "Operation: F.U.T.U.R.E.", this is Zigzagged. The villain was apparently able to create the BadFuture not only by doing this ''intentionally'', but by keeping in regular contact with her future self. On the other hand, Numbuh Four's future self did speak to him after giving the rest of the team vital information needed to SetRightWhatOnceWentWrong, but he never told them who he was. [[spoiler:And given what is revealed in the SeriesFinale, it would seem that this future timeline is undone completely, and Numbuh Four's future is changed.]]
* ''WesternAnimation/SonicSatAM'': In "Blast to the Past", Sonic and Sally travel back in time and meet their past selves. Nothing bad happens from this and the past selves don't know who they really are.

* This is literally the case with fermions[[note]]a class of fundamental particle that includes electrons, also can refer to anything with an odd number of fermions[[/note]]. Two of the same fermion cannot exist in the same place. It's inverted with bosons, which are more likely to exist in the same place than predicted by chance. Time travel is unnecessary here, since the way quantum physics works, any two particles with the same properties are, in fact, the same particle. Interestingly, since chemistry is based on this fact, somehow getting ahold of truly different electrons would be disastrous, if far from reality-destroying.
* Using special relativity and the last comment, it's easy to make this situation happen in real life, and show that it's not a problem. By moving one particle faster, it can be made to pass through time at a different rate. Since both are the same particle, but more time passed for one, then one of them is a future version of the same particle. They interact as normal, showing that they are, in fact, the same particle, and being JustOneSecondOutOfSync is not an attribute that exists on a fundamental level.