Just like a {{Flashback}} or deja vu, but the suddenly resurfacing memory comes from the character's AlternateSelf in an AlternateUniverse.

Often caused by the fact that LoveTranscendsSpacetime.

Just like a Flashback or FlashForward, a Flash Sideways can be in a narrow or wide definition. The narrow definition is when a character has one of these three because of a resurfacing memory, some kind of supernatural connection or whatever. The wide definition is when a show or movie itself has one of the three. In the wide definition, they don't have to be [[JustifiedTrope justified]] on the contrary, the entire point can be to give the viewers information that the characters don't have access to. Flashbacks and flashsideways have the same name both in the wide and in the narrow definition. Flash-forwards are so called in the wide definition only. In the narrow definition they are known as [[Film/MinorityReport precogs]] or {{Seers}}.

The trope codifiers are the movie ''Film/SlidingDoors'' and the final season of ''Series/{{Lost}}'', both of which use this trope extensively.
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!!Examples:

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[[folder: Anime And Manga ]]

* In ''VisualNovel/HigurashiWhenTheyCry'', the nature of [[spoiler: the arcs]] is revealed when a character briefly flashes back to the first arc, ''Onikakushi-hen'', [[spoiler: which was in an AlternateUniverse]].
* In ''Manga/KatekyoHitmanReborn'', Byakuran experiences this frequently as a special ability, allowing him to gain knowledge from other timelines where he's taken over the world and use that to conquer the reality that the manga/anime actually takes place in.

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[[folder: Comic Books ]]

* The ''Comicbook/{{Flashpoint}}'' timeline's Traci 13 gets glimpses of her life in the main DCU.
* In ''ComicBook/AstroCity'' a character has visions of his wife who was erased when the timeline changed.

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[[folder: Fan Fics ]]

* In the ''Film/StarTrek'' fanfic ''[[https://www.fanfiction.net/s/9795158/1/Written-in-the-Stars Written in the Stars]]'', Fem!Kirk is shown flash backs of her counterpart's past, showing said counterpart's relationship with Spock. Halfway through the story both counterparts experience a FlashSideways, showing what's happening in the Alternate Reality of the Mirror Universe.
* Several characters from the [[MirrorUniverse Negaverse]] in the WesternAnimation/DarkwingDuck fanfiction, ''FanFic/NegaverseChronicles'', end up dreaming about the [[MirrorUniverse other universe]] with Darkwing Duck and the Fearsome Five.

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[[folder: Film ]]

* In ''Film/SlidingDoors'', the main character sometimes gets a sense of Deja Vu when she meets someone important to her in the other timeline but she has never met in her own timeline.
* In ''Film/DonnieDarko'', as suggested by the hand waving in the last scene, some characters retained vestigial memories of the parallel timeline.

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[[folder: Literature ]]

* ''Literature/{{Discworld}}'' series:
** In ''Discworld/MonstrousRegiment'', a Black-Ribboner vampire suffering from serious coffee withdrawal while serving as a soldier in the Borogravian army suffers "flashsides" of the Vietnam war.
** Also invoked in ''Discworld/{{Jingo}}'', where Sam Vimes and his AlternateSelf grab each other's "Dis-Organizers"; Main TL Sam keeps getting harrowing messages from the Alt-Dis-Organizer of repeated disasters from the ParallelUniverse where he stayed in Ankh-Morpork.
** Happened to Granny Weatherwax in ''Discworld/LordsAndLadies'', due to "circle time" causing the "walls" between alternate realities to become thinner. "And here I thought my mind was going! Hah, it was going and fetching!"
* Happens repeatedly in ''Literature/{{Everworld}},'' where the main protagonists have each been split into two people, one version living in UsefulNotes/{{Chicago}} and one trapped in the eponymous alternate universe. Whenever their Everworld-selves lose consciousness or fall asleep, their spirits return to their Earth-counterparts, giving a sudden and unexpected flash of memories. Which can be quite disturbing, considering the kinds of things that happen to them in Everworld...
* In one of the ''Literature/{{Animorphs}}'' books, Jake asks the RealityWarper Drode to make it so that they never learned about the alien invasion and got their powers. It turns out Cassie is a temporal anomaly who can subconsciously remember the original timeline and whose very presence causes the altered one to fall apart. It's heavily implied that this was the reason that the [[SufficientlyAdvancedAlien Ellimist]] arranged for Cassie to become an Animorph in the first place, as the Drode accuses him/it? of "stacking the deck" by her being on the team.
* In ''Franchise/TheDarkTower'', saving the life of one of the characters creates a temporal paradox that causes two of the characters to suffer from chronic FlashSideways to the original, unaltered timeline. This is a very negative condition which is slowly driving them both insane.
* In Creator/SergeyLukyanenko's ''[[Literature/NightWatch The Last Watch]]'', Anton once again encounters Yegor, who was central to the first novel. Yegor mentions having a vivid dream of being Anton's son and other things not being quite the same. This was a clever way of the author including the films (which are loosely based on the books) into the canon universe without upsetting anyone. This way, it may appear that the films take place in an AlternateReality or a dream. A similar comment is made when Ilya recounts a dream he had, which mirrors [[BigLippedAlligatorMoment one of the most random and over the top scenes]] in the first movie.
* Murakoks from ''Literature/LabyrinthsOfEcho'' live like this -- sometimes the contact with alternate selves intensifying to HiveMind level. The drawback is that actual travel between the worlds is not an option, since moving from one's correct place in the network means immediate insanity. They also [[BlueAndOrangeMorality tend to see things in the different light from most humans]]: Koba from World of the Rod is a Beggars' Foreman who refused to join Secret Investigations in order to keep some sort of balance between his selves, [[BeethovenWasAnAlienSpy Koba from World of the Spider]]... you know ''[[JosefStalin whose]]'' pseudonym it is, right?
* In the Literature/DiogenesClub story "Literature/{{Swellhead}}", Richard Jeperson gets tangled with an alternate reality in which he died taking down a supervillain in the 1970s. At one point, he starts having flashbacks to things that happened to the alternate Richard -- and then a brief burst of flashbacks from a large number of other alternate Richards (most of them shout-outs to author Creator/KimNewman's many other alternate history stories).

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[[folder: Live Action TV ]]

* In one episode of ''Series/{{Fringe}}'', Olivia Dunham occasionally has visions of the AlternateUniverse prominent in the series.
* In the sixth season of ''Series/{{Lost}}'', the characters' [[AlternateSelf Alternate Selves]] in the AlternateTimeline gradually start remembering the lives of their counterparts in the timeline of the first five seasons. [[WordOfGod Team Darlton]] [[TropeNamer coined the trope's name]], although they used it in the wide definition, to describe the idea of intercutting events in different universes ( la [[FlashForward flash-forwards]] and [[FlashBack flashbacks]]). [[spoiler:It turns out that it was a big subversion, though: The alternate timeline was, in fact, [[FlashForward just the afterlife.]] Though it's debatable whether the alternate timeline being the afterlife and a parallel world are mutually exclusive.]]
* In ''Series/StarTrekTheNextGeneration'', Guinan seems to be aware of alternate universes, as demonstrated in "Yesterday's Enterprise". [[HandWaved Explained]] in the ExpandedUniverse as a by-product of her leaving a part of herself in The Nexus, which allows a person to access any place or time. In her case, it gives her nothing more than intuition about the way things "should" be.
* ''Series/DoctorWho'':
** In "Turn Left", an alternate universe gets built around Donna Noble, where because she [[ForWantOfANail turned right instead of left one day]] the Doctor is dead, the Earth has gone to Hell and all of reality is going to be destroyed. She doesn't remember the original time-line, but when she restores things to normal she has flashes of memory from the Turn-Left verse.
** After "The Big Bang", the Doctor, Amy, Rory, and River all seem to retain memories of the cracked universe.
** The next season finale "The Wedding of River Song" shows yet another alternate universe, where [[spoiler:all of history happens at once. Some people are able to retain ideas of what reality should be like- for example, Amy draws pictures of all of the things she remembers. For example, she knows she has a husband she loves very much, she knows he's Roman...but she can't remember his name or what he looks like, so doesn't realise he's the soldier "Captain Williams"]].
* ''Series/BuffyTheVampireSlayer''. Averted in the Wishverse Universe where the Scoobies, split between vampire and human vigilante "White Hats", brutally slaughter each other without remorse. Most notably Wishverse!Buffy -- an emotionless cold-blooded killer -- meets her [[StarCrossedLovers Star Crossed Lover]] Angel and is unimpressed, not even reacting when [[TakingTheBullet he gives his life to save her]] from Vampire!Xander.
* In the ''Series/{{Supernatural}}'' episode [[Recap/SupernaturalS02E20WhatIsAndWhatShouldNeverBe "What Is And What Should Never Be"]], Dean has flashes of what is going on in the real world, seeing the djinn's other victims, while he is in the Wishverse.

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[[folder: Video Games ]]

* The plot of ''{{VideoGame/Marathon}} Infinity'' sees the player-character continually flashing across multiple timelines until he finds one in which an EldritchAbomination can be defeated. By defeating it in one timeline, it gets defeated in all of them. For bonus points, the game begins in a timeline in which the events of the previous game, ''Marathon 2: Durandal'', never happened.

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[[folder: Web Comics ]]

* In ''Webcomic/ElGoonishShive'', Tedd's dream of himself as Lord Tedd during the "Sleepy Time" mini-arc may be this. Later, [[spoiler:magically enforced]] to Ellen in "Second Life".
* In ''TheDreamer'' Beatrice tends to get these a lot, especially in the 21st century timeline.
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