Fakin' MacGuffin

A possible outcome of a Hostage for Macguffin situation: The hero reluctantly hands over the MacGuffin, the audience facepalms, the villain gloats... and then it turns out that the MacGuffin the hero handed over was actually a fake.

The effect of the fake MacGuffin will vary: sometimes it will destroy the villain when he tries to use it, or even bestow the mysterious boon he was trying to steal on the hero instead; sometimes, of course, it will merely do nothing at all.

Hopefully, the existence of a phony MacGuffin will have been set up in advance, and some effort will be expended to explain why the villain doesn't immediately notice it's a phony.

Not to be confused with Mock Guffin, when it turns out in the end that the MacGuffin everybody has been chasing is a fake, and there is no "real" MacGuffin.

Usually, the audience doesn't find out that the villain's got a phony until the villain does (or until the hero reassures his sidekick), so unmarked spoilers abound.

Generally not related to Egg MacGuffin.

Anime and Manga

 * In Mahou Shoujo Lyrical Nanoha Striker S, there is a struggle over a case containng a relic. At the end, the Numbers seem to make off with it, to shock from all sides. Then, after they've left... Teana and Subaru reveal that, actually, they took the relic out of the case and hid it in Caro's hat -- all the Numbers have is an empty case.

Comic Books

 * Buck Godot Zap Gun for Hire: PSmIth: After Buck is obliged to hand over the MacGuffin he was hired to guard, he reveals that he had a copy made and it was the copy he handed over. Or did he hand over the real one, and this is the copy...?
 * Villainous example: The Joker pulls one of these to get his hands on the all-powerful "worlogog" in a Justice League story.
 * Green Lantern does this periodically. Somebody demands that he hand over his ring, and he appears to do so, sometimes with the villain even creating constructs... and then once he skedaddles, he reveals to the reader and/or a person nearby wondering why he isn't concerned that the ring he handed over was actually a construct and the real ring is still on his finger and invisible, and that the fake ring will fade away after an hour or so.

Film

 * In National Treasure Ben was able to slip the baddies a souvenir Declaration of Independence instead of the real thing.
 * Also happens in National Treasure 2.
 * In the climax of Short Circuit, Johnny 5 makes a copy of himself to keep from getting destroyed by the army.
 * Hudson Hawk: Eddie is forced to put together the pieces of the critical element of Leonardo da Vinci's gold-making machine (which the villains have been pursuing throughout the movie). However, he leaves out one crucial part, which eventually causes the machine to malfunction and explode.
 * In The Score, Nick Wells (anticipating a double-cross) swaps out the sceptre he was hired to steal for they axle the used as stand-in during their trial runs, meaning that Teller ends up when nothing when he does betray him.
 * At the end of Sneakers, when the NSA attempts to confiscate the MacGuffin, Marty hands it over, telling them "It doesn't work. It never did" -- and, after they've gone, reveals to his colleagues that he's kept the true MacGuffin, the one component that makes the thing work.
 * In Super Mario Bros, Mario holds some shoelaces in a way that suggests that he has the meteorite piece pendant, and taunts Koopa with it. Only when Lena attempts to merge the worlds with the real rock does Koopa realize Mario was tricking him.
 * Played with in the first Mission Impossible movie: The protagonist hands a disk over, then convinces his rival that it was a fake so he throws it away, then picks it up from the trash, revealing that it was real after all.
 * In Indiana Jones and the Last Crusade, Elsa presents Donovan with one of the false Grails, leading to his demise.
 * Ronin has the protagonists attempting to steal a silver briefcase. The first time they try to steal it, it turns out to be a decoy with a bomb inside.

Literature

 * In Seven Ancient Wonders by Matthew Reilly, the villain is trying to complete a ritual that will give him awesome power, in which a box of sand from his homeland will play a key part. The hero is a globetrotting adventurer who always carries with him, as a reminder of home, a jar of his own home soil. I don't think I need to spell out what happens, do I?
 * Explorers of Gor. Long story short(er), Shaba the Cartographer has an Invisibility ring captured from the Kurii which he was supposed to deliver to the Priest-Kings, but he (seemingly) turned traitor and was going to deliver an exploding ring instead & return the real ring back to the Kurii. Except he kept the invisibility ring for himself to explore Darkest Gor. When both the Priest-King representative and the Kurii find him after his voyage of discovery they fight, and the Kurii capture the ring. But it turns out the get the wrong ring. Earthshattering Kaboom ensues.
 * In one of The Three Investigators books the bad guy wants a journal that the boys have which was written in the mid 1800s, with potential clues to a Buried Treasure. Jupiter hands it over, then after the bad guy leaves he reveals that he only gave up the oilskin cover of the journal, having taken the pages out first.
 * In The Dresden Files short story "The Warrior" Harry leaves a fake Sword of The Cross on the front seat of his Beetle. The 'Sword' gets stolen, then Harry uses a tracking spell to find it again.
 * The fake Horcrux at the end of Harry Potter is an example of this gone wrong: Regulus Black replaces one of the series' MacGuffins with a fake in an attempt to bring down Voldemort, but instead it backfires on our heroes when they gather the MacGuffins in their own attempt to bring down Voldemort.
 * Redwall: As part of a plan to recover the tapestry of Martin, Jess distracts Cluny with a crude fake. He realizes this as soon as she gives it to him, but by then it's too late.

Live Action TV

 * Doctor Who
 * In The Keys of Marinus, the Doctor and his companions find a fake Key along with the five real ones. When the villain tries to get the Keys off them at the end of the story, they give him four real Keys and the fake, and he gets blown up when he tries to use them.
 * Also used in The Daleks' Master Plan, where the Doctor steals the power source of the Daleks' Doomsday Device. By the time they catch him, he's made a fake to offer them in exchange for his freedom.
 * In the Blood Ties episode "Norman", the heroes Vicki and Henry are forced to give a magic dagger to the demonic villain Norman when he kidnaps Vicki's secretary and holds her hostage. Norman needed the dagger to complete a spell to release the uber demon Asteroth into the world. However, unknown to the audience, Vicki and Henry had first taken the dagger to a priest to have it blessed before they gave it to Norman, so that when he used it, his spell of summoning failed and he was sucked back down to Hell.
 * Burn Notice: Michael doesn't trust a thief he's forced to work with, because she's framing him for a murder, so he gives her a canister full of fake nerve gas (tear gas And Some Other Stuff), and takes the real stuff, keeping her under the illusion that it's fake.

Video Games

 * In Sonic Adventure 2, Sonic tries to slip Eggman a fake Chaos Emerald, but Tails blows the secret.
 * Devil May Cry 2. Big Bad Arius is after four artifacts known as Arcana so he can revive the Demon World's former king (Argosax the Chaos) and absorb his power. Although she puts up a valiant fight, Lucia is defeated and held hostage by Arius, and Dante (who has all of the Arcana in his possession) comes to her rescue; Arius asks for the items in exchange for Lucia. Things get wonky from here: Dante is quick to hand over the Arcana, but then hands Arius his ass. Arius then manages to escape with the Arcana by playing up Lucia's temporary damsel in distress status as he blows up his stronghold. When Arius initiates the resurrection ceremony, he fails. Dante then coolly walks onto the scene and reveals that he played Arius like a chump by switching the Arcana Medaglia with his own lucky coin, playing this trope straight again.
 * In the first of the Thief games, which are entirely based around stealth, this is how you defeat the final boss. As he is preparing his ritual, you sneak into his altar room and swap out The Eye for a fake eye to cause his ritual to blow up in his face instead of summoning chaos to the world.
 * In Paper Mario: The Thousand-Year Door, the Crystal Star in Poshley Heights is displayed in plain sight and is the only one like that. Beldam and her sisters, who had spent the previous three days trailing Mario, nabs it before Mario could get to it. It turns out this one is a fake; Inspector Pennington, who runs the display, actually let them take it to get them away from there, then leads Mario to the real one.
 * In Mario & Luigi: Superstar Saga, the heroes create a false Beanstar to give to Fawful and Cackletta... but they don't fall for it, instead discarding the fake and taking the real one by force.
 * Drakensang: After recovering the Dragon's Eye from Castle Grimtooth, the party is confronted by the Big Bad Malgorra and her servants, who are holding hostage the young son of Traldar, the owner of the castle. In order to save him, you must hand over the Eye to Malgorra, but is later revealed that it was a fake all along and that Traldar kept the real one on him.
 * Metal Gear Solid 3 Snake Eater: EVA seduces Snake, steals the microfilm you've been chasing for the whole plot and reveals herself as a Chinese double agent. Then it's revealed that Ocelot was a CIA double agent, switched Snake's microfilm with a fake, and took the real one back to the CIA himself.

Web Original

 * Grandmaster of Theft sees this happen in the second episode with a villain example. Narcissa rigged her challenge by not bringing the real Undine's Tear in the first place just in case she lost.
 * Cassidy goes on to use said fake to her advantage to Out Gambit Narcissa back with her own weapon. However, the second time she tries this trick with Deus, he's more than aware she was going to do it and shots the fake before requesting the real one.

Western Animation

 * In the American Dad episode "Black Mystery Month", Steve does this with a jar of peanut butter. Stan tried to do it, too, but failed, because he thought they would switch themselves via magic.
 * Happens a couple of times in Lilo and Stitch: The Series. At one point, Stitch switches a capsule containing an invisible invisibility-experiment for an empty capsule to fool Gantu, and in another, a shape-shifted guest star is substituted for a shape-shifting experiment.
 * On Space Stars, in the Teen Force short "Wordstar", Moleculad turns an asteroid into a fake Wordstar to give to Uglor after Elektra acquires the real one.