Stolen MacGuffin Reveal

Sister Margaretta: Reverend Mother, I have sinned.
Sister Berthe: I, too, Reverend Mother.
Reverend Mother: What is this sin, my children?

[The nuns look at each other, then reveal from under their robes the distributor and coil they have removed from the Nazi's cars, stopping them from catching the Von Trapp family.]

—The Sound of Music

There's a MacGuffin everyone is after and the Big Bad has just managed to snatch it right out of the Hero's hands and is laughing as they make their escape, with The Hero left in a bit of a Heroic BSOD. But wait! The Comic Relief, team thief, or even a less obviously sneaky character suddenly brightens the mood by revealing that they've just done the same—stole the MacGuffin right out from under the villain's nose as he made his escape, too. Being a cheeky larcenist, the thief may even taunt the retreating villain with it, though they usually replace the stolen MacGuffin with a flawless decoy, worthless rock, mocking note, or other means of taunting the villain. If the MacGuffin is in a case, the thief may do a Satchel Switcheroo.

As the page quote shows, the Stolen MacGuffin Reveal can also be used to other ways, such as removing engine parts to stop a villain in their tracks from getting the MacGuffin or another objective. Also worth noting is that the hero/villain roles can be reversed.

May or may not result in an "Everybody Laughs" Ending.

Examples of Stolen MacGuffin Reveal include:

Anime and Manga

Death Note, sort of. At the end, Near steals the Death Note and replaces it with a fake to prove that Light is Kira.

Comic Books

There's an early Elf Quest story where Cutter and Skywise get captured by Picknose the troll and his family because Picknose believes the pommel of Cutter's sword is the key to a treasure chamber. After escaping, Cutter is upset that the sword he got from his father is now ruined - until Skywise reveals that he stole back the pommel and hid it in his mouth (explaining why he seemed so tight-lipped during their escape).

Film

In The Fifth Element, the case carrying the four stones needed to stop The Evil is fought over intensely. After villain Zorg finally gets it, it's amusingly revealed to be empty, much to Zorg's frustration. Their guardian, Plava Lagoona, showed how seriously she takes guarding them by revealing she hid them in her stomach.
Hudson Hawk. While Eddie is assembling the three parts of the da Vinci device that turns lead into gold, he secretly keeps one of the parts. When the Villains try to use the incorrectly combined device, it explodes and kills them.

Eddie: What would happen if I didn't put that crystal together exactly right? Let's say I left out this piece of mirror here. Would that be bad?
Anna: Real bad.
Eddie: Good.

In Muppets from Space, Man in Black K. Edgar Singer threatens the aliens with a BFG, which then cheerily announces "Please load weapon" in a Computer Voice.

Kermit: That was a close one.
Bobo the Bear: Not as close as you think, my friend. (holds up futuristic gun clip) "Please load weapon"...

In Sneakers, it's revealed that the good guys win because Martin surreptitiously stole a piece of the McGuffin device in the end.
In The Sound of Music, the Von Trap family manages to escape pursuing Nazi's because the nuns at the convent they were hiding in "sinned" by removing engine components from their cars.
At one point in Schindler's List, factory workers steal the firing mechanism from a pistol belonging to Those Wacky Nazis.
Star Trek III the Search For Spock After Kirk and Co steal the Enterprise the Smug Captain of the Excelsior brags about how this will be the shortest hijack in history because his ship will quickly overhaul the old relic. Scotty, meanwhile, has nicked some rather important bits of it's engine, leaving the Excelsior floating helplessly in space.

Scotty: Here, Doctor, souvenirs from one surgeon to another. I took them out of her main transwarp computer drive.
McCoy: Nice of you to tell me in advance.

Sherlock Holmes 2 has Holmes reveal he stole Moriarty's red notebook with all his funds and assets. Though he doesn't hold the original, he gloats in having replaced it with one that he creatively drew a fisherman eaten by a shark in.
Aladdin: Variation—immediately after thinking he'd escaped from the Cave of Wonders, Jafar discovered he no longer had the lamp; cue Big No. Then it's revealed, when Aladdin thinks Jafar got away with it, that Abu had stolen it back from him.

Literature

Crown of the Russian Empire ends with The Dragon Ovechkin thinking he threw the Russian Imperial Crown into the sea to avoid its repossession by Chekists... until the Lovable Rogue Naryshkin reveals that what Ovechkin threw was an empty bag, and the crown is safely with him, and hands it to the heroes.
Roald Dahl's short story The Hitchhiker has the title character swipe a policeman's notebook after the protagonist gets a speeding ticket.
Played with in A Night in the Lonesome October: one of the supporting characters attempts one of these to give his side an advantage in the final showdown, but he misjudges it with the result that his side comes close to losing before he explains what he did.
Terry Pratchett's Thud! ends on this note. A small army of dwarfs and trolls are scouring a cavern for a MacGuffin, but Nobbes, with the infamous Nobbes' Sidle already snagged it.
In Treasure Island, Silver's crew finally reach the place where Captain Flint buried his treasure, only to find that it had been dug up already. Turns out a marooned Ben Gunn had found it years earlier and kept it hidden inside a cave.
The Hobbit, in which the last few chapters revolve around Bilbo having stolen the one specific piece of treasure that the increasingly cantankerous and misanthropic Thorin really wanted to find amongst the vast hoard of the defeated dragon Smaug. Bilbo uses it to convince Thorin to make peace with the humans.

Western Animation

The Simpsons: Bart steals the keys to the Fighting Hellfish treasure from Mr. Burns while hugging him, asking to be brought along.