->''"Sixty years from now, it will be a child's toy. But today, it's the most powerful weapon on earth."''
-->-- '''Vandal Savage''', ''WesternAnimation/JusticeLeague''

The reverse of the ButterflyOfDoom, the Timeline-Altering [=MacGuffin=] is a nondescript item from the future that, if left in the past, will bring about an AlternateTimeline. This can be something that contains information about the future, such as a history book, for instance. It could also be a future technology that [[GivingRadioToTheRomans someone in the past decides to reverse engineer]]. The item will probably become a MacGuffin pretty quickly.

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!!Examples:

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[[folder:Anime and Manga]]
* Possibly used in ''MahouSenseiNegima'' when [[spoiler:[=Chao Lingshen=] produces what may or may not be a future copy of the Springfield family register]] as the ultimate party-breaking item. Whether it was real or not is irrelevant, since it was destroyed shortly after. She did, however, bring a number of real pieces of information and technology from the future as well.
** In chapter 349, [[spoiler:it makes a return later and is revealed to be... blank. The future was changed, so the family register has yet to be filled out.]]
** It could also make a [[spoiler:FridgeHorror; the family register didn't written because up until his death, ''Negi never made a family.'']] Of course, you could also argue that [[spoiler:Negi ''didn't'' write his family register for whatever reason, but it brings another UnfortunateImplications; what kind of life [[LawfulGood Negi]] had that he didn't have time to register his family, or worse ''had to not register his family?'']]
* The "Whispered" of ''LightNovel/FullMetalPanic'' are privy to "[[BlackBox Black Technology]]" -- devices they should be unable to design for decades or even centuries. It's extended their ColdWar clear into the 21st century.
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[[folder:Comics]]
* In ''Comicbook/TheSandman'', people are digging up a bunch of those from an archaeological dig in Crete. Sadly [[spoiler: some of them are explosive.]]
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[[folder:Film]]
* The Gray's Sports Almanac in ''Film/BackToTheFuture Part II'', which allowed Biff Tannen to become wealthy through using the information from the 2015 Gray's Sports Almanac that his future self brought to him in 1955 to make bets on the outcomes of sporting events.
** In the first draft of ''Back To The Future'' Marty revealed to 1955 Doc that all his crazy inventions could be cheaply powered with Coca-Cola (ItMakesSenseInContext... [[ItRunsOnNonsensoleum well, almost]]) so when he traveled back to 1985 he actually ended in an alternate reality where everything was a {{Zeerust}} 50's rendition of the future, with hovering cars and robots everywhere powered by Coca-Cola. The dystopic part? [[ALittleSomethingWeCallRockAndRoll No Rock and Roll!]]
* The Jet Engine in ''DonnieDarko'' -- also an example of an (Un)StableTimeLoop and a TemporalParadox.
** Probably one of the soonest 'futures' though- less than one month later.
* The arm and CPU from the first Terminator in ''Film/TheTerminator'', left behind in the 1980's, brought about the rise of SKYNET. [[TimeyWimeyBall Well, sometimes.]] In any event, since the Terminator was sent back in time ''by'' SKYNET, this is also a StableTimeLoop...until it's broken by the destruction of the items in ''Film/Terminator2JudgmentDay'', which doesn't stop SKYNET from rising again in ''Film/Terminator3RiseOfTheMachines'' (where the novelization even implies that despite the destruction of the items, what had already been researched was aprehended by the military leading to SKYNET's creation). Yeah, TimeyWimeyBall.
* ''Franchise/StarTrek'':
** In ''Film/StarTrekIVTheVoyageHome'', Scotty teaches the 20th century plastic maker how to make "transparent aluminum", justifying it with the argument, "We know it was invented around this time, but we don't know the name of the man who invented it. It could have been him." The original script had Scotty saying that he knows that the manager would eventually go on to "invent" it, therefore giving him the formula created a StableTimeLoop.
** The 2009 ''Film/StarTrek'' film:
*** Played with the trope [[spoiler:by a future Spock showing Scotty an equation that he would eventually create -- however earlier in the film the reason that future Spock is there is explicitly stated to be an AlternateUniverse, not a StableTimeLoop. Scotty had not only already invented the formula, but had tested his theory before. It went wrong, leading to his assignment at the remote outpost we see, but this could be part of the AlternateUniverse. Spock only shows Scotty a ''revised'' version of his theory, bringing his own discovery earlier into the timeline.]]
*** The nigh-unstoppable warship belonging to the villain is also an example. In its own time period it is merely a mining vessel. Its ability to [[spoiler: blow up planets]] is due to something that they stole [[spoiler: from Spock]].
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[[folder:Literature]]
* In ''ArtemisFowl: The Eternity Code'' Artemis steals some fairy technology (which is at least 20 years ahead of human tech), builds a computer out of it, and offers to sell it to a ruthless multi-billionaire. Said billionaire then steals the computer and shoots Butler. [[spoiler: He gets better.]]
* ''Literature/AConnecticutYankeeInKingArthursCourt'' by Creator/MarkTwain. It goes from LighterAndFluffier to DarkerAndEdgier in a serious MoodWhiplash, thanks to the author's personal issues at the time.
* In the sequel to ''TheBookOfNightWithMoon'', a spin-off of the ''YoungWizards'' series, a book on modern-day engineering gets sent back into an AlternateUniverse [[VictorianLondon Victorian England]], enabling the British Empire to rapidly develop many forms of modern-day technology. Including nukes.
* This trope is played around with in ''ThePendragonAdventure''. Generally, taking an item from one territory to another is said to cause disaster and allow Saint Dane to win. This first occurs in ''The Merchant of Death'', the very first book, when Bobby ignores this warning and [[spoiler: gives the Milago tribe all the necessary parts to make an atomic bomb]]. Saint Dane is sometimes shown as doing this as well, such as in ''Black Water'' where he [[spoiler: uses a poison from another territory to try and poison the locals]], but in the very same novel, the protagonists use [[spoiler: the antidote from the same territory as the poison]] to foil his plot. It goes so far as to have Bobby give the people of Ibara [[spoiler: weapons from Quillan to defeat an army of Quillan dado robots, who themselves were attacking Ibara on skimmers from Cloral]].
* In the ''{{Dragonlance}} Twins'' novels, Caramon bringing a volume detailing events back from [[AfterTheEnd a very dark future]] was the reason Krynn did not falter into an AlternateContinuity where it was utterly destroyed
* The ''Literature/SixteenThirtyTwo'' series is about a small town from West Virginia sent back to the central Germany during the UsefulNotes/ThirtyYearsWar, so almost every object in the town is a TimelineAlteringMacGuffin, particularly books telling of events beyond the point at which they arrived in seventeenth century Europe.
* Done in Alfred Bester's ''Time and Third Avenue.'' A young lawyer accidentally gets a copy of the almanac from the far future year of 1985 and a timecop stops him before he opens it. The lawyer says all right, you can have the almanac: using it to speculate on stocks or bet on elections would be cheating, and I'm sure I can have a great career without cheating... only, I wish I could have some reassurance that the world won't end in a nuclear holocaust. So the timecop gives him a hundred dollar bill, one of the 1980 series. He can't spend it in his time, but he feels that he's well paid [[spoiler: when he reads the name of the Secretary of the Treasury from the bill... and it's him.]]
* In HarryTurtledove's ''TheGunsOfTheSouth'', in which time-traveling Afrikaners help the South win the AmericanCivilWar, we have ''The Picture History of the Civil War'', a simple history book. Its major influence is in revealing how the Afrikaners lied to the Confederates, particularly by claiming that the world of the future is an endless racial war between blacks and whites, as well as disproving the Confederates' belief that they would be VindicatedByHistory for holding African slaves. This is what drives the Confederates to openly oppose the Afrikaners, as well as leading to the passage of a bill that provides for the gradual, compensated release of the slaves.
* "Mimsy Were the Borogoves" is a classic sci-fi short story by Lewis Padgett. A scientist doing an experiment in time travel realises he doesn't have anything to send back, so he grabs some of his children's educational toys and sends them back to 1943 (when the story was written) where they educate a brother and sister how to move into another dimension, which they do before their father's horrified eyes.
* Occurs in "Mr. Was" where the main character previously wrapped a sandwich in newspaper. This newspaper had stocks from the future on it when he went to the past. The man who later became his grandfather found this paper and used it to get rich.
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[[folder:Live-Action TV]]
* In ''The Hanged Man'' episode of ''{{Journeyman}}'', a digital camera Dan accidentally left in the early 1980s causes technology to leap far forward when he returns to modern times -- including a family member being wiped from existence because of a "nanotech accident" on the day of his conception.
* A non-time travel version occurs in the ''Series/StarTrekTheOriginalSeries'' episode "A Piece of the Action": A pre-Prime Directive starship 100 years prior had inadvertently left behind a book on Chicago gangs of the 1920s, which caused the civilization in question to develop into the original PlanetOfHats, with a culture based around 1920s gangsterism. After the crew has fixed the entire planet, [=McCoy=] leaves his communicator behind, alarming Kirk who worries that this trope will take effect. "In a few years they could be demanding a piece of ''our'' action!"
** One of the comics actually referenced that event when various representatives were at Admiral Kirk's trial including one from that planet, with the only result being that the representative simply gives it back to [=McCoy=] after saying how he'd left it behind. (They had still kept their original gangster society, though the profits had been invested in things like schools, libraries, and hospitals. What Kirk hadn't anticipated was that as the new boss, they had saved 10% of the take as Kirk's personal cut.)
** The ''[[Series/StarTrekDeepSpaceNine Deep Space Nine]]'' writers at one point considered doing an episode that revisits the planet and finds that all the inhabitants are now dressing up as Kirk and Spock. This episode was to be a Star Trek 30th anniversary celebration, and was dropped in favour of "Trials and Tribble-ations". Considering how good "Trials and Tribble-ations" turned out, they probably made the right choice. The concept was later picked up in a comic.
** The 1989 tie-in book ''[[http://memory-alpha.org/wiki/The_Worlds_of_the_Federation The Worlds of the Federation]]'' had the same idea. When contact was re-established sometime around the Next Generation time period, the entire planet had restructured their society around TOS-era Starfleet and extrapolated enough technology to build a working starbase and apply for Federation membership.
** The NES ''Star Trek: 25th Anniversary'' game starts out with the Enterprise being pulled into unknown space by a wormhole. When they finally reach Federation space again, they figure out that the wormhole was a side effect of the gangster planet blowing itself up. The last mission requires you to go back in time to retrieve the communicator and prevent the explosion/wormhole.
** There was also the Next Generation comic issue "A Piece of Reaction" which follows the same plot (more or less).
* ''Series/StarTrekVoyager'':
** "Future's End" has a 29th Century timeship that was sent back to 1967. Interestingly, the 29th century technology helps create the "holo-emitter" the Doctor uses for the rest of the series, so the timeship was a TimelineAlteringMacGuffin to the 24th Century as well.
** The final episode leaves the possibility of this trope rather alarmingly dangling overhead. Future Janeway comes back over a decade to bring the crew home, decking out the ship in all kinds of future tech and eventually infecting the Borg Queen with a super nasty future virus. Now, given the Borg's ability to adapt, one can speculate that if they manage to overcome that virus, they would then have adapted to technology and programming the Federation hasn't yet invented. Not only that, but they had already assimilated her shuttle from the future by then, including the armor and the torpedoes. In the ExpandedUniverse novels, specifically ''StarTrekDestiny'', this comes back to bite them.
* In the ''Series/DoctorWho'' episode "The Long Game", Adam attempts to leave one of these for himself in the past by recording historical information from 197,988 years in the future on his mother's answering machine. The Doctor finds out. He's not happy. Ironically, Adam himself ends up at risk of becoming one. He has his own brain upgraded in the future, to interface with the computers of the time. Now whenever someone snaps their fingers near him a little port on his forehead opens up. The Doctor mentions that Adam has to lead a quiet life and not draw attention to himself, or he risks [[TheydCutYouUp being dissected]] for the future technology in his skull. At the end of the episode, his mother snaps her fingers while talking to him.
* In ''Series/StarTrekEnterprise'', when [[spoiler:stranded]] in the 29th century, Archer finds a book about [[BigBad the Romulan Star Empire]]. Luckily, Daniels is there to stop him from reading it. When they finally encounter the Romulans in a later episode (but do not see them), Archer mentions the book title.
* In ''LoisAndClark'', after the first time Tempus was defeated, he was left in the past, where he wrote a diary a man would eventually use to become wealthy by investing in oil, plastic and computers. Later, that man's [[TheUnfavorite unfavorite son]] used the diary to blackmail {{Superman}} into stealing from the man's other son. According to this VillainOfTheWeek, Tempus was either a man from the future or a fortuneteller good enough to put [[FortuneTeller Nostradamus]] to shame.
* ''{{Misfits}}'' has an episode where an elderly Jewish man with time travel powers tries to go back and kill Hitler. [[HitlersTimeTravelExemptionAct That ends badly]], but Hitler manages to retrieve the man's cell phone from him before the man gets sent back to his time, leading to a future where the Nazis have taken over Britain due to technological superiority. [[TooDumbToLive Why the man went back in time with both his cellphone and a letter explaining he was going back in time to kill Hitler (instead of leaving the letter behind in case he failed to return) is not explained]].
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[[folder:Radio]]
* In the ''Series/DoctorWho'' [[AudioPlay/BigFinishDoctorWho audio drama]] "Colditz", it's discovered that their accidental appearance at Colditz Castle goes horribly wrong, and Nazis from the 1960s reveal they got the ''TARDIS''. The twist is that [[spoiler: the TimelineAlteringMacGuffin isn't the TARDIS, but a CD Player which leads to the getting to the concept of LASER quicker than they were supposed to, thus having them win the rocket race, making the V missiles more deadly than they were and eventually letting the TARDIS end up in their hands.]]
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[[folder: Video Games ]]
* While not exactly a mundane item from the future, the titular ''[[Franchise/TheElderScrolls Elder Scrolls]]'' serve a similar function. They were written by a god and contain records of history: All that has happened, all that will happen (usually in the form of "If X happens, then Y and Z will happen, in that order"). They are extremely hard to read, causing blindness if read too often, but they are used by the emperor to secure the wellbeing of his empire, guiding his course of action.
** In ''[[VideoGame/TheElderScrollsVSkyrim Skyrim]]'' the player gets to read one themselves. If used near a temporal rift it lets them see an important piece of the past; used anywhere else it strikes them blind.
*** What's more, they are absolutely and utterly immutable, such that they can change history, just by being read. In ''[[VideoGame/TheElderScrollsIVOblivion Oblivion]]'' the ultimate thieves guild quest involves stealing one in order to break a Daedric curse.
* Another non-time travel version is in ''VideoGame/PredatorConcreteJungle''. The game starts in 1920s Chicago, where the Predator accidentally leaves some of his technology behind. Cut to modern-day, and the technology has become way advanced, with {{Hollywood Cyborg}}s and [[IWantMyJetpack flying cars and all that fun stuff.]]
* In ''SDGundamGGeneration DS'', the cast of ''Anime/TurnAGundam'' travels back in time to try and prevent the apocalyptic "Dark History" from coming to pass. Unfortunately, [[Anime/MobileSuitGundam Gihren Zabi]] gets his hands on the Dark History data, which allows him to produce an army of super-mecha equipped with knockoffs of the Turn A's GrayGoo weapon.
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[[folder:Western Animation]]
* ''WesternAnimation/JusticeLeague'' villain Vandal Savage sends a laptop containing the history of WorldWarII to himself in the 1930s. With his advanced knowledge, [=WW2=]!Savage easily outwits the Allied forces, deposes Hitler and sets his sights on world domination. The present!league minus Batman (given RippleEffectProofMemory by being protected by Green Lantern's power when it was sent back) must go back in time to prevent this.
* In a ''SuperFriends'' episode, the Superfriends are able to find the Legion of Doom by traveling to the year 7000 and finding a future almanac, which they simply look up the year when the Legion of Doom took over the future (the past?) in the year 3984. By the end of the episode they keep it.
* One episode of ''WesternAnimation/AmericanDad'' had Stan go back in time to prevent Creator/JaneFonda from ruining Christmas (in his opinion). A tape of disco's greatest hits is accidentally left behind, where it's discovered by the past version of Roger the Alien, who becomes insanely rich and then goes broke riding the rise and fall of the disco trend.
** In the end of the episode, the family complains that Roger is whining about disco, ''again'' (something he never did before in the series.)
* ''WesternAnimation/BeastWars'' features the Voyager Disk, which initially only helped Megatron find earth, but was eventually revealed to contain both photos from the planet's [[TimeTravelTenseTrouble present, in the future, which is the past]], and a [[FlingALightIntoTheFuture message from G1 Megatron]] ordering [[TimeTravelTenseTrouble future decepticons to go into the past to change the present.]] Both were used in attempts to MakeWrongWhatOnceWentRight, with varying degrees of success.
* In ''WesternAnimation/TransformersRescueBots'', Doc Greene's robot, Dither, ends up being one of these. [[OlderThanTheyLook Doctor Morocco]] gets his hands on Dither in 1939, which leads to a BadFuture.
* ''WesternAnimation/BigGuyAndRustyTheBoyRobot'': When BGY-11 was accidentally sent to the past, it was found by English soldiers who learned how to control it. They renamed it "Iron Jack" and used it to stop UsefulNotes/TheAmericanRevolution. Because nobody knew everything needed for its maintenance, it eventually blew up but the damage was done. As a result of this, nobody developed BGY-11 or anything else that could have stopped the alien invasion BGY-11 did at the beginning of the series in the original timeline. Dwayne Hunter and Rusty then had to travel back in time to recover it and set history right.
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