Often caused by the fact that Love Transcends Spacetime.
Just like a Flashback or Flash Forward, 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 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 precogs or Seers.
This has some overlap Ripple Effect-Proof Memory, the difference being that in this case the characters have no awareness of the split between timelines or universes and are unable to figure out what exactly is going on.
The trope codifier is the final season of Lost, which uses this trope extensively.
Split Timelines Plot is a sub-trope where a single diverging event creates two (or more) timelines, which the work proceeds to jump between (a la Sliding Doors). Usually, the characters are unaware of any different timelines in that trope.
Contrast with Allohistorical Allusion, where the reference to another universe (usually our own) is more of an in-joke by the writers than anything significant.
- In Higurashi: When They Cry, the nature of the arcs is revealed when a character briefly flashes back to the first arc, Onikakushi-hen, which was in an Alternate Universe.
- In Katekyō Hitman Reborn!, 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.
- In Steins;Gate, only Okabe has a fully-Ripple Effect-Proof Memory, but others can start to gain memories from alternate timelines when traits of it are described to them.
- The Big Finish Doctor Who episode, "Jubilee" has the Sixth Doctor getting in sporadic mental contact with a version of himself from an alternate timeline, who has been through pure hell to put it mildly. He is at first confused by it, and then utterly horrified once he figures out what is going on.
- Another Big Finish Doctor Who episode, "Zagreus", has the Eighth Doctor, due to the influence of an Eldritch Abomination, mentally perceiving adventures by his analogue in an alternate timeline, heavily implied to be the Eighth Doctor Adventures. He is quite shaken by the experience.
- The Flashpoint timeline's Traci 13 gets glimpses of her life in the main DCU.
- In Astro City a character has visions of his wife who was erased when the timeline changed.
- In House of M: Spider-Man, Peter Parker, whose life is perfect, goes mad from the nagging feeling he doesn't deserve it, and obsessively writes an imaginary journal where everyone hates Spider-Man, Uncle Ben and Gwen Stacy are dead, and it's all his fault. Setting the self-loathing aside, he appears to be tapping into his "real" history somehow.
- Before Watchmen says that Doctor Manhattan flashes between multiple realities. It was originally just two: One where he got superpowers and one where he didn't but they eventually split into dozens.
- In the Star Trek (2009) fanfic 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 Flash Sideways, showing what's happening in the Alternate Reality of the Mirror Universe.
- Several characters from the Negaverse in the Darkwing Duck fanfiction, Negaverse Chronicles, end up dreaming about the other universe with Darkwing Duck and the Fearsome Five.
- In Split Second, Twilight and Sparkle (alternate versions of each other) can pull time sideways, allowing the two timelines to intersect and interact. Interesting, in that rather than being a narrative element, it is a physical phenomenon in-universe.
- In Donnie Darko, as suggested by the hand waving in the last scene, some characters retained vestigial memories of the parallel timeline.
- Discworld series:
"And here I thought my mind was going! Hah, it was going and fetching!"
- In Monstrous Regiment, a Black-Ribboner vampire suffering from serious coffee withdrawal while serving as a soldier in the Borogravian army suffers "flashsides" of the Vietnam war. As it gets worse people nearby start experiencing these hallucinations too.
- Also invoked in Jingo, where Sam Vimes and his Alternate Self grab each other's "Dis-Organizers"; Main TL Sam keeps getting harrowing messages from the Alt-Dis-Organizer of repeated disasters from the Parallel Universe where he stayed in Ankh-Morpork.
- Happened to Granny Weatherwax in Lords and Ladies. Granny spends much of the story worried she's going senile when she keeps recalling false memories, but after a conversation with Ridcully about Ponder's ramblings about alternate universes, she realizes what's really going on. Due to "circle time" causing the "walls" between alternate realities to become thinner (and threatening Lancre with an invasion of The Fair Folk), she's actually picking up thoughts from alternate versions of herself.
- Also by Terry Pratchett, in The Dark Side of the Sun, journeys through Interspace cause the traveller to see alternate realities.
- Happens repeatedly in Everworld, where the main protagonists have each been split into two people, one version living in 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 Animorphs books, Jake asks the Reality Warper 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 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 The Dark Tower, saving the life of one of the characters creates a temporal paradox that causes two of the characters to suffer from chronic Flash Sideways to the original, unaltered timeline. This is a very negative condition which is slowly driving them both insane.
- In Sergey Lukyanenko's 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 Alternate Universe or a dream. A similar comment is made when Ilya recounts a dream he had, which mirrors one of the most random and over the top scenes in the first movie.
- Murakoks from Labyrinths of Echo live like this — sometimes the contact with alternate selves intensifying to Hive Mind 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 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, Koba from World of the Spider... you know whose pseudonym it is, right?
- In the Diogenes Club story "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 Kim Newman's many other alternate history stories).
- Crashing through the barrier at the edge of the universe causes David Brinkley from Superfolks to see possible versions of how his life could have turned out if he'd made different decisions.
- In one episode of Fringe, Olivia Dunham occasionally has visions of the Alternate Universe prominent in the series.
- In the sixth season of Lost, the characters' Alternate Selves in the Alternate Timeline gradually start remembering the lives of their counterparts in the timeline of the first five seasons. Team Darlton coined the trope's name, although they used it in the wide definition, to describe the idea of intercutting events in different universes (à la flash-forwards and flashbacks). It turns out that it was a big subversion, though: The alternate timeline was, in fact, just the afterlife. Though it's debatable whether the alternate timeline being the afterlife and a parallel world are mutually exclusive.
- In Star Trek: The Next Generation, Guinan seems to be aware of alternate universes, as demonstrated in "Yesterday's Enterprise". Explained in the Expanded Universe 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.
- Doctor Who:
- In "Turn Left", an alternate universe gets built around Donna Noble, where because she 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 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".
- Buffy the Vampire Slayer. 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 Star Crossed Lover Angel and is unimpressed, not even reacting when he gives his life to save her from Vampire!Xander.
- In the Supernatural episode "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.
- In The Flash (2014), Cisco's first vibe is of him being killed by Eobard Thawne, which happened in a previous timeline, changed by Barry running back in time.
- In the Red Dwarf episode, Skipper, Rimmer uses a device called a quantum skipper to switch places with versions of himself from alternate universes.
- Tokyo Dark: New Game+ is this trope; Ito's hallucinations allow her to make lucid dreams of other timelines, ie other runs of the game. Also, the Collector and the Historian, both mysterious powerful characters, discuss the implications of multiple timelines after each ending and the results of that ending, all mentioning that they are side paths to the "inevitable end" (that being the Becoming ending).
- The plot of Marathon Infinity sees the player-character continually flashing across multiple timelines until he finds one in which an Eldritch Abomination 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.
- One of Jenny Everywhere's abilities is to share thoughts and memories with her alternate selves in other universes.
- The Myth Arc of 12 oz. Mouse is kicked off when the protagonist starts having mysterious dreams like this; visions of a whole life and family he doesnt remember ever having. As he starts looking into the dreams, he suddenly realizes that he has some fairly large gaps in his memory... such as the fact that he doesnt even remember ever coming to the city in the first place. The ending seems to suggest that the flashes are his real memories and everything hes been experiencing in the city is part of some sort of Lotus-Eater Machine.