Time travel and temporal anomalies can be messy and unpleasant. When time starts getting out of whack, anything can happen. People can be born over and over, meet themselves, relive the same day, or just screw up all their relationships. Here are all the ways people get tangled in their own timelines in science fiction.
"A Little Something For Us Tempunauts" (several days)
In this classically-cynical Philip K. Dick story, the whole world gets caught in a time loop, living the same few days over and over again.
Groundhog Day (24 hours)
Bill Murray's jerk-weatherman is condemned to repeat the same day over and over until he figures out exactly what he's supposed to do in every situation.
X-Files "Monday" (24 hours)
The world gets stuck in a Groundhog Day-style loop, with Scully and Mulder caught up in a bank robbery. Eventually the only person who knows they are all in a loop convinces Mulder, and then eventually he is able to make himself remember the loop and foil the bank robber.
Day Break (24 hours)
The premise of this sadly short-lived series is that Detective Brett Hopper has to relive the same day over and over until he figures out who framed him for murder.
Twilight Zone "Shadow Play" (24 hours)
A man is arrested, tried, and executed on the same day. He knows he's trapped in a nightmare, but it continues to repeat, with the "actors," i.e. judge, cellmates, etc. switching roles the second time around. The source for X-Files Monday.
12:01 (tv movie) (24 hours)
This time, HR employee Barry is the only one who recognizes that his office is stuck in a "time bounce," in this version for a whole day, thanks to some sinister experiments. Barry has to get to the bad guy in order to break out.
Star Trek TNG "Cause and Effect" (12 hours or so)
In this episode, the Enterprise gets caught in a "temporal causality loop" because of a space-time anomaly. The loop ends in the Enterpise's destruction until Data figures out how to send himself a message that will break the ship out of the loop.
12:01 (short film) (1 hour)
Myron Castleman repeats the same hour over and over. He's unable to break out even through suicide.
Source Code (8 minutes)
Colton Stevens lives the same eight minutes over and over again in the body of another man, while he tries to prevent a terrorist bombing of Chicago
Doctor Who (a few minutes, usually)
Time loops are a major fact of life on Doctor Who — but most notably, the Doctor puts everybody into a time loop to stop a major catastrophe, in "The Armageddon Factor." And the Doctor himself is caught in a "chronic hysteresis" in "Meglos," causing Romana, K9 and him to relive the same moment over and over.
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The First Fifteen Lives of Harry August (variable lifetime)
Harry August lives his whole life over and over, returning to infancy but remembering all his previous lives. Eventually, he discovers that the end of the world is approaching, and he tries to stop it. With many "Ourouborans," who repeat their lives over and over, the novel avoids paradoxes by describing the time loop as millenia long, with everyone living one life per loop.
Life After Life (a variable lifetime)
Ursula Todd also repeats her life along multiple timelines, and eventually begins to remember those repetitions. Although she begins in the same year, changing timelines result in changing dates of death. In her first incarnation, she dies during her own birth.
Replay (25 years)
Jeff Winston dies of a heart attack in 1988, but then wakes up 25 years earlier as his 18-year-old self. He lives his whole life over, but dies again of a heart attack. He repeats the loop, and he meets others experiencing the same phenomenon, but it gets smaller and smaller each time.
Timequake (10 years)
In this Kurt Vonnegut novel from 1997, the entire world lives the period 1991-2001 a second time.
The Man Who Folded Himself
Daniel Eakins inherits a "timebelt" and uses it to interact with himself in different settings and different times. Let's just say that "folding yourself" is a euphemism for something else here.
Assassins for an organized crime syndicate kill targets sent from the future, until eventually they must "close the loop" and kill their own future selves.
"By His Bootstraps"
In this Heinlein short story, the main character, Bob, encounters multiple versions of himself as they pass through a "time gate" from the future. Each time Bob tries to do something differently, he discovers that possibility is already accounted for, with another version of himself.
After being chased by a mysterious man with a bandaged face, Hêctor takes refuge in a time machine that sends him back one hour. He becomes the mysterious figure who chased himself, and the Hêctors continue to multiply.
Star Trek (2009)
Spock and a Romulan miner Nero are sent back in time through a black hole, where Spock eventually meets his younger self.
Bill and Ted's Excellent Adventure
Bill and Ted from the future meet themselves in the Circle K parking lot and convince themselves to travel into the past.
Thanks to everyone who mentioned this one! In this film, people not only meet their past selves, they whack their past selves on the head and impersonate themselves. It's a whole thing.
Spoiler alert — in the new season of this Canadian time-travel show, things have gotten a bit confusing. Some people are very narrowly missing meeting themselves, including one encounter that's already turned very unpleasant for all concerned.
People are constantly meeting their younger selves and getting their timelines tangled on Doctor Who. In particular, the Doctor and his companions encounter their future selves in "The Space Museum," the Doctor and Jo meet themselves in "Day of the Daleks," the Brigadier has a close encounter with himself in "Mawdryn Undead," Amy Pond crosses paths with her older alternate self in "The Girl Who Waited," Kazran Sardick meets himself as a young boy in "A Christmas Carol," and the Doctor is always meeting his past selves.
Protagonist Dana is summoned back in time by her ancestors, and she is forced to intervene in order to assure her own conception. Unfortunately, her ancestors include both slave owners and their slaves.
Chuck Palanhniuk's protagonist may (or may not) have traveled back in time, met his own great-great-grandmother, and become his own ancestor.
Quantum Leap "The Leap Between the States"
A leap takes Sam Beckett to the Civil War, and into the body of his grandfather. Although this breaks the rules of Sam's quantum-leaping, it is explained in part by Sam's close genetic relation.
Back to the Future
Marty McFly accidentally messes up his parents' budding romance and has to fix it... or he'll never be born. Which would be bad. For rock'n' roll, at least.
Expiration Date by Duane Swierczynski
The protagonist is transported back to the 1970s, but only as an observer. He tries to figure out how to intervene and prevent his father's murder.
Masters of the Universe
As a reward for helping He-Man defeat Skeletor in Eternia, when Julie is sent home through a dimensional portal, she arrives well before she left, and is able to prevent her parents from getting on a plane that will crash.
A son discovers that his short-wave radio allows him to communicate with his father, 30 years in the past, and he uses this to avert a tragedy.
Billy Pilgrim witnesses events in his life before they happen, as either an effect of being an inmate in an alien zoo, or an effect of the trauma he suffered during WWII.
Time Traveler's Wife
Henry DeTamble, the eponymous time traveler, zooms around within the timeline of his life, and eventually his wife's life, as an effect of a genetic disorder, "Chrono-Impairment."
Linda Hanson (Sandra Bullock) lives the days of the week in which her husband is killed out of order. Although she doesn't save her husband, she does slightly alter the events so that, when she lives the day of her husband's accident a second time, she is pregnant.
Star Trek TNG: "Parallels"
Worf discovers that he is quantum-fluxing through a space-time fissure between multiple timelines with subtle but significant differences. He is the only one who is aware of the changing situations.
Electromagnetic radiation causes both people and objects, like the island, to become unstuck and bounce around within their own timelines. In some cases, this can become very nasty unless you find your "Constant."
In the novel by Robert J. Sawyer and the TV show that adapted it, everybody blacks out and sees a vision of their own future — but you can try and change what happens. Emphasis on "try." (Thanks to everyone who mentioned this one!)
Meeting people out of order is a way of life for the Doctor, especially in the new series. The Second Doctor meets a much later version of the Brigadier, in "The Five Doctors." Also, this phenomenon defined his whole relationship with River Song. Also, a future Doctor met his past companions in "The Impossible Astronaut." And he has a very jumbled encounter with Madame Pompadour in "The Girl in the Fireplace." And so on.
Peggy Sue Got Married
At her 25-year high school reunion, Peggy Sue faints. She wakes up as her high-school self, but doesn't do anything differently.
Hot Tub Time Machine
The protagonists travel back in time to 1986 and arrive in their younger bodies, where they try not to change anything in the past — except for Lou.
"Jeffty is Five"
Jeffty stays five while everyone around him ages. The world in his immediate vicinity also stays the same. Radios broadcast programs from the 1940s, and he sends away for a decoder ring from 1945 in the 1970s.
Something Wicked This Way Comes
A sinister magic carousel causes people to grow older as they ride forward, younger as they ride backward.
Jorge Luis Borges' stories suggest time loop and crossing timeline paths. In "The Garden of Forking Paths," the forking paths are forking timelines, "Three Versions of Judas" and "Tlön, Uqbar, Orbis Tertius" suggest alternate timelines.
Characters in Hades and in the Inferno are condemned to repeat the same activities for eternity, while in Purgatory, like in Groundhog Day, once someone repents, they get to break out of the repetition. In the Decameron, a man must eternally hunt and dismember the woman who refused to marry him.
What examples of time-whackery did we miss? Let us know!