One World War II era spy film was already in the can when the atom bomb was dropped. It was yanked from release, to have its dialog somewhat reworked. A secret process, integral to the atom bomb, was replaced for — whatever MacGuffin the spies had looked for earlier.
The neutrinos in 2012 (it's mostly unintentional due to bad writing); not only do the planet's neutrinos "mutate and heat up the earth" and lead to "the end of the world", they never get another mention or fixed yet everything works out.
In The Adventures of Buckaroo Banzai Across the 8th Dimension, once it's established in the opening scenes that the perfected oscillation overthruster allows travel to the titular dimension, much of the movie is largely devoted to the good guys keeping it out of the Red Lectroids' hands (as they want to free their imprisoned peers from the dimension as part of their return to Planet 10), resulting in a Hostage for MacGuffin situation in the third act. In the end Emilio Lizardo/Lord John Whorfin loses patience and decides his original, imperfect overthruster will suffice. It doesn't work, nor does their attempted escape to Planet 10 after that. A government official who wants the overthruster for military applications actually finds it in Penny's purse in the denouement, but only temporarily thanks to Scooter.
The Archer: The ledger that proves Bob's bribing the judge to send girls into his reform camp become this, with Lauren and Rebecca stealing it during their escape and Bob and Michael relentlessly chasing after them mostly so they can get it back.
Artemis Fowl has the Aculos — a mysterious fairy object of unimaginable that Opal Koboi demands in exchange for Artemis Fowl Sr.'s safe return.
Atomic Blonde has the List, a microfilm hidden inside a wristwatch which contains the names of every undercover operative (on both sides of the Cold War) active in Berlin.
James Cameron's Avatar. The Unobtanium is supposed to help make interstellar travel more practical, but there is no further space travel past the point when Jake arrives on Pandora shortly after the film begins. The exact reason for the conflict between the humans and Na'vi is basically irrelevant for the rest of the plot.
The X-5 Unit in Beavis and Butt-Head Do America, a bio-weapon that is said to be able to kill five states in five days. And it's hidden in Beavis' pants. Beavis and Butt-Head's TV counts as this as well.
The rug in The Big Lebowski. Not only is it interchangeable as a plot device, the plot gets started because his first rug is ruined and he steals another random rug to replace it.
Birds of Prey (2020) has the Bertinelli diamond, which contains the codes to said extinct mob family's lost fortune. Cassandra Cain steals it without realizing what it actually is, making her a target of everyone else who wants it, which kicks off the main plot.
In The Blade Master, the hero Ator is called upon by his old mentor to protect something called the geometric nucleus, out of fear that a villain would capture it and use it as a weapon. The film is extremely unclear about what it is ("everything and nothing"), though its name and other details point to it being a Fantastic Nuke. In any case, its only power over the story is to heighten the supposed stakes if Ator doesn't defeat the evil warlord who seizes the mentor's castle.
Blood and Black Lace: Everyone at the fashion house wants the diary belonging to the first victim, Isabella, because it exposes all of the shady stuff they've been up to, including cocaine dealings and an abortion. It is destroyed by one of the models halfway through the film for that reason.
The invented 'letters of transit' in Casablanca, though unusually they do get used at the end of the film.
The unopened box from Cast Away. The movie ends with Chuck finally delivering it to its destination after four years... only to find no one at the destination.Word of God joked that it was a waterproof satellite phone. A deleted scene from the script would have shown it contained two bottles of salsa verde.
In Children's Party at the Palace, the MacGuffin is Her Majesty's handbag, which contain her reading glasses needed for her to read the closing speech (And the speech happens to be in the bag as well). All the storybook protagonists go on a quest to find the missing...eventually stolen handbag before its too late.
The Green Destiny sword in Crouching Tiger, Hidden Dragon is a classical MacGuffin. While it does see a lot of combat and is a very good sword, its value is mostly ideological. It doesn't have any special abilities except of withstanding considerable abuse and being perfectly crafted.
Lampshaded in The Departed: "Our target: microprocessors. Yes, those. I don't know what they are, you don't know what they are, who gives a fuck?"
In the 1979 film The Double McGuffin (narrated by Orson Welles), a group of precocious children (including Lisa Whelchel find a briefcase full of cash and run afoul of Ernie Borgnine and Lyle Alzado.
The jailbreak in Down by Law. We never find out how they got out, and it doesn't matter, because the movie is more concerned with the relationships between the characters (see also Noodle Implements).
The stoner-flick Dude, Where's My Car? has two; first, the titular car, which serves primarily as a plot device to lead our half-baked heroes into zany misadventure after zany misadventure, and second, the Continuum Transfunctioner, a very mysterious and powerful device (Its mystery is only exceeded by its power.) being covertly fought over by two different alien races (which represent themselves as hot chicks and creepy Nordic dudes, respectively), a fight that the protagonists slowly find themselves caught in the middle of.
The Nightshade device in 6. It's only mentioned once or twice and has something to do with satellites.
Subverted with the God's Eye in 7. It sounds a lot like a usual MacGuffin Â a device that can hack into anything and trace anyone anywhere Â but once it's recovered, it's used almost immediately to find Deckard. It's used again during the climax by Jakande to track down Ramsey and keep her from locking him out of it.
In Gentlemen Explorers, everybody is searching for the Infinity Pistol: a magical gun that never needs reloading.
When the screenplay for Good Will Hunting was published as a book, director Gus Van Sant wrote a preface in which he admitted that Will's math talents were a MacGuffin: he doesn't solve a math problem the details of whose solution affect the plot (otherwise, the movie would be more a science-fiction story about the invention of fusion power, or whatever).
Done for humor in Help!; Ringo Starr is given the ring of the goddess Kaili, which he can't get off and which various villains and bad guys are trying to get. One Mad Scientist comes out with the classic MacGuffin line: "With a ring like that, I could—dare I say it?—rule the world!!"
In Mel Brooks' High Anxiety, which contains parodies of numerous Hitchcock films, the lead character (who is terrified of heights) is checking into a hotel when the receptionist informs him that though the hotel had reserved him a lower-level floor, "a Mr. MacGuffin called and requested we change it to the 17th floor." Though MacGuffin is probably a reference to the villains stalking the main character, the name is never mentioned again.
This was almost certainly an intentional reference to the technique itself, as Hitchcock, who is parodied in the film, popularized the term MacGuffin.
The gold in The Italian Job. Less so in the remake, if we go by the 'only counts if it's not spent' rule. Much is made about how various character plan to/do spend it.
It's a Mad, Mad, Mad, Mad World is based around a bunch of fools trying to locate and claim a hidden stash of $350,000. That's $2.73 million dollars in 2015, by the way. The movie ends with the money being lost making it entirely irrelevant.
Juno MacGuff from Juno invokes this trope symbolically with her last name, signifying that after she becomes pregnant she becomes the object of nearly everyone's (often intrusive, unhelpful, and/or unsolicited) attention. Almost everyone has their own opinion on what she should do with her child and her body without respecting her own agency and choice on the matter.
Loose Cannons (1990) has a rare Nazi porno starring Adolph Hitler himself. This object is being searched by the Nazis, Israelis, main character detectives, and the FBI.
The titular object from The Maltese Falcon. It gets the characters together, pits them against each other, but turns out to be worthless.
In Satan Met a Lady, an earlier movie adapted loosely from Dashiell Hammett's original novel featuring a young Bette Davis in the Femme Fatale role, the Falcon was replaced by Roland's trumpet, a legendary horn said to be filled with priceless jewels.
The Reality and Power stones are both sought after by the villains of their respective shows, and in the climaxes used to empower them, but they could easily be replaced by any other powerful thingamajig.
The Mind stone is somewhat more important than the others, used to make both Ultron and Vision sapient in Avengers: Age of Ultron, but could be removed with little difficulty.
The soul stone is the worst of it, existing solely for Thanos to search for in Avengers: Infinity War and create some angst. It is never even used for any noticeable purpose other than a split second in the fight between Thanos and Dr. Strage in order to pick out the real Strange from a bunch of doppelgangers.
The Galaxy on Orion's belt in Men in Black. It's a miniaturized galaxy disguised as the bell-charm on the dead jeweler's cat, Orion — nothing to do with the constellation after all.
Metallica: Through the Never has roadie Trip risking life and limb to deliver a duffel bag containing "something very important." The movie never shows what's in it, even when Trip opens it up to see. He's apparently shaken by it, though. The film ends on a shot of it, still closed.
Mission: Impossible has a disc as the primary MacGuffin, though it was clearly defined as being a list of undercover IMF agents.
Mission: Impossible III features Ethan Hunt trying to keep a nasty weapons dealer from acquiring "The Rabbit's Foot", a small cylindrical kajigger that's assumed to contain some sort of biological weapon (though it's never explicitly stated as such). At the very end of the film, as Hunt leaves to enjoy his honeymoon, he asks his boss just what "The Rabbit's Foot" was, but his boss says he'll only tell him if he stays with the IMF. They all have a good laugh about it, and the movie ends. Shockingly some film reviewers (professional critics!) expressed outrage that they didn't get to find out what the all-important item was, suggesting unfamiliarity with the trope.
Mike's mother in My Own Private Idaho — the driving force for the plot is him trying to find his long-lost mother, but in the end he never does, even though he goes as far as Italy to find that she's just left. No MacGuffin, No Winner perhaps?
A classic Alfred Hitchcock example is the "government secrets" that motivate the action in North By Northwest (1959). The audience only sees a quick shot of the microfilm, and we never have any idea what was on it in the first place, or why it was important.
One of Alfred Hitchcock's earliest examples of a MacGuffin is the uranium sand that Claude Rains was smuggling in wine bottles in Notorious ("A vintage sand" is what Cary Grant called it). When studio execs told Hitchcock that movie audiences wouldn't understand why the uranium sand was so important, Hitchcock answered, "Then we'll make it uncut industrial diamonds. It doesn't matter what it is, as long as the villains want it. That's the MacGuffin, that's the motor that drives the plot."
Milton's stapler in Office Space. It doesn't see much use as a stapler, but it is important to the subplot involving Milton and Lumbergh (which eventually does tie back to the main plot).
Several in each Pirates of the Caribbean movie, usually with Jack's compass or the Black Pearl coming into play at some point.
At World's End had Calypso and the Nine Pieces of Eight. They would have been actual coins, but the 1st Pirate Council was broke. Instead... Ammand the Corsair's pewter brandy goblet, Barbossa's wooden eye, Capitaine Chavelie's lucky red Ace of Spades, Mistress Ching's glasses, a jade gemstone woven with silk for Sao Feng, Gentleman Jocard's tobacco cutters, Cptn Jack Sparrow's Siamese coin woven into beads, Sri Sumbajee's snuff box, and a glass shank that Villanueva won a fight with.
Pulp Fiction: The suitcase with the glowing and mysterious contents for Vince and Jules. It's an homage to the 1955 movie Kiss Me Deadly, whose suitcase originally housed a superweapon — a nuclear device. A popular fan speculation is that it houses Marcellus' soul. Tarantino has made it very clear that he neither knows nor cares what was in the case. He has said that in hindsight he wishes he had not even included the glow, since that narrows down the possible contents, vague as it may be.
In the first draft of the film, it was diamonds. When filming began, they decided "diamonds" was too generic and overused, and decided that we'd never see what was inside the case.
The 1964 Chevy Malibu in Repo Man, which contains something radioactive in its trunk.
In Road to Rio, there are the mysterious Papers that have no bearing on the plot besides having an interesting Safe Cracking scene. Lampshaded when Bob Hope and Bing Crosby say that "the world must never know" their contents. At the end, when the papers have been recovered and they're about to be read, they get torn up instead, since they've served their dramatic purpose.
The silver case in Rōnin. The fact that the main characters' employers refuse to tell them what's inside the case is a minor plot point. We never find out either, and Robert De Niro's character isn't even interested in the case: he was just using it as a way to get to IRA leader Seamus so he could assassinate him.
The Spanish Prisoner revolves around a secret and valuable industrial "Process" its protagonist has invented.
The Pick of Destiny in... Tenacious D in The Pick of Destiny. Tenacious D need it to become the greatest rock band ever and win enough money to pay off their rent, and never use it outside of a dream sequence.
Titanic (1997) is framed around the search for a diamond called Le CÂur de la Mer (The Heart of the Sea/Ocean), which is quickly forgotten until the end of the story, when its owner throws it overboard so no one can have it.
Trust No 1: The NSA had been searching for a drive containing top secret military intelligence data. It also turns out to have access to the treasury.
What's Up, Tiger Lily? has several factions out to kill to possess the perfect egg salad recipe, stolen from a potentate who tells our hero "It is written that he who makes the best egg salad shall rule over heaven and earth. Don't ask me why egg salad, I have enough aggravation."
The Wizard of Oz has Dorothy's ruby slippers, which she needs to keep away from the Wicked Witch of the West. It's never explained what they do, but Glinda points out, "Their magic must be very powerful, or else she wouldn't want them so badly." Though in the end they are used to bring Dorothy home.