Timeline-Altering MacGuffin

""Sixty years from now, it will be a child's toy. But today, it's the most powerful weapon on earth.""

- Vandal Savage, Justice League

The reverse of the Butterfly of Doom, the Timeline-Altering MacGuffin is a nondescript item from the future that, if left in the past, will bring about an Alternate Timeline. 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 someone in the past decides to reverse engineer. The item will probably become a MacGuffin pretty quickly.

The former Trope Namer Gray's Sports Almanac is from Back to The Future Part II. (Whenever referring to this trope, be sure to pronounce it like Marty did when he confronted Biff in the hot tub ("Gray's... Sports... Almanac"). It's fun!)

Anime and Manga

 * Possibly used in Mahou Sensei Negima when 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,
 * The "Whispered" of Full Metal Panic are privy to "Black Technology" -- devices they should be unable to design for decades or even centuries. It's extended their Cold War clear into the 21st century.

Comics

 * In The Sandman, people are digging up a bunch of those from an archaeological dig in Crete. Sadly

Film

 * The former Trope Namer was Gray's Sports Almanac in Back to The Future 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 (It Makes Sense in Context... 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? No Rock and Roll!
 * The Jet Engine in Donnie Darko -- also an example of an (Un)Stable Time Loop and a Temporal Paradox.
 * Probably one of the soonest 'futures' though- less than one month later.
 * The arm and CPU from the first Terminator, left behind in the 1980's, brought about the rise of SKYNET. Well, sometimes. In any event, since the Terminator was sent back in time by SKYNET, this is also a Stable Time Loop...until it's broken by the destruction of the items in the second movie, which doesn't stop SKYNET from rising again in the third. Yeah, Timey-Wimey Ball.
 * In Star Trek IV, 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 Stable Time Loop.
 * The 2009 Star Trek film played with this trope
 * It should be noted that
 * 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  is due to certain modifications, including Borg technology.
 * They also rely heavily on ambush; with surprise on their side they take out entire fleets in moments, but in a stand-up fight the Enterprise can hold her own by shooting down its improvised torpedoes.

Literature

 * A Connecticut Yankee in King Arthur's Court by Mark Twain. It goes from Lighter and Fluffier to Darker and Edgier in a serious Mood Whiplash, thanks to the author's personal issues at the time.
 * In the sequel to The Book of Night with Moon, a spin-off of the Young Wizards series, a book on modern-day engineering gets sent back into an Alternate Universe Victorian England, enabling the British Empire to rapidly develop many forms of modern-day technology. Including nukes.
 * This trope is played around with in The Pendragon Adventure. 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 . Saint Dane is sometimes shown as doing this as well, such as in Black Water where he , but in the very same novel, the protagonists use   to foil his plot. It goes so far as to have Bobby give the people of Ibara.
 * The trope is reversed in the Dragonlance Twins novels. Caramon bringing a volume detailing events back from a very dark future was the reason Krynn did not falter into an Alternate Continuity where it was utterly destroyed
 * The 1632 series is about a small town from West Virginia sent back to the central Germany during the Thirty Years War, so almost every object in the town is a Timeline-Altering MacGuffin. A stupid king of England got his hands on history books and began executing people who would be responsible for the English Revolution years before it happened, thus earning some enemies years early and . Cardinal Richelieu got his hands on history books, realized a problem with his current policy that he hadn't noticed before, started an impressive Xanatos Gambit, and purchased America. And everyone is trying to reverse engineer 20th-century weapons. The storyline follows the altered timeline so it's impossible to say exactly how this will change things, but a safe guess would be "a lot".
 * Of course the protagonists are Genre Savvy enough to realize what's going on, and go to great lengths to secure their remaining books.
 * 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, of course, but he feels that he's well paid
 * In Harry Turtledove's The Guns of the South, in which time-traveling Afrikaners help the South win the American Civil War, 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 Vindicated by History 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.
 * Ocurrs in "Mr. Was" where the main character previously wrapped a sandwhich 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.
 * Ocurrs in "Mr. Was" where the main character previously wrapped a sandwhich 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.

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 Star Trek: The Original Series 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 Planet of Hats, with an entire 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.
 * The 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 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).
 * Star Trek: Voyager has a 29th Century timeship that was sent back to 1967. Interestingly, the 29th century technology helps create the "holoemitter" The Doctor uses for the rest of the series, so the timeship was a Timeline-Altering MacGuffin 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.
 * And in the Expanded Universe novels, specifically Star Trek Destiny, this does come back to bite them.
 * In the Doctor Who 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 is 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 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 Star Trek: Enterprise, when in the 29th century, Archer finds a book about the Romulan Star Empire. Luckily, Daniels is there to stop him from reading it.
 * In Lois and Clark, 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 unfavorite son used the diary to blackmail Superman into stealing from the man's other son. According to this Villain of the Week, Tempus was either a man from the future or a fortuneteller good enough to put Nostradamus to shame.

Radio

 * In the Doctor Who 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.

Video Games

 * While not exactly a mundane item from the future, the titular 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 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 Oblivion the ultimate thieves guild quest involves stealing one in order to break a Daedric curse.
 * Another non-time travel version is in Predator: Concrete Jungle. 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 Cyborgs and flying cars and all that fun stuff.
 * In SD Gundam G Generation DS, the cast of Turn a Gundam travels back in time to try and prevent the apocalyptic "Dark History" from coming to pass. Unfortunately, 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 Gray Goo weapon.

Western Animation

 * Justice League villain Vandal Savage sends a laptop containing the history of World War II to himself in the 1930s. With his advanced knowledge, WW 2!Savage easily outwits the Allied forces, deposes Hitler and sets his sights on world domination. The present!league minus Batman (given Ripple-Effect-Proof Memory 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 American Dad had Stan go back in time to prevent Jane Fonda 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.
 * Beast Wars features the Voyager Disk, which initially only helped Megatron find earth, but was eventually revealed to contain both photos from the planet's present, in the future, which is the past, and a message from G1 Megatron ordering future decepticons to go into the past to change the present. Both were used in attempts to Make Wrong What Once Went Right, with varying degrees of success.