Flash Sideways

Just like a Flash Back or Deja Vu, but the suddenly resurfacing memory comes from the character's Alternate Self in an Alternate Universe.

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 resurfing 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.

The trope codifiers are the movie Sliding Doors and the final season of Lost, both of which use this trope extensively.

Anime and Manga

 * In Higurashi no Naku Koro Ni, the nature of  is revealed when a character briefly flashes back to the first arc, Onikakushi-hen,.

Comic Books

 * 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.

Film

 * In the movie Sliding Doors, 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.

Literature

 * In the Discworld novel 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.
 * 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, 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!"
 * In The Doomfarers of Coramonde, the mage's scrying produce a vision of a incomprehensible metal beast that the reader may recognize as a tank. Later this "monster" is conjured, making a group of american soldiers (the tank's crew) the new protagonists in the fantasy world.
 * 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 titular 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 Reality 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 pseudonim it is, right?

Live Action TV

 * 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).
 * 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.
 * In Doctor Who, 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-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.
 * Buffy the Vampire Slayer. Brutally 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.
 * Buffy the Vampire Slayer. Brutally 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.

Web Comics

 * In El Goonish Shive, Tedd's dream of himself as Lord Tedd during the "Sleepy Time" mini-arc may be this. Later, to Ellen in "Second Life".
 * In The Dreamer Beatrice tends to get these a lot, especially in the 21st century timeline.