Shell Game

Two or more identical things are shown, but only one of them is significant. Either the viewers or some of the characters know which one it is until they get mixed up. They do not come into view already mixed. The most common variation involves covering an item such as a peanut with a shell and shuffling it with other shells, and playing cards may be substituted. (The card variation is also known as "find the lady" or three-card monte.) The next most common variation has a hero and a look-alike get in a fist-fight while in sight of another hero, and results vary.

Briefcases are also extremely susceptible to this. However, in those situations, the audience always knows which one is which because they never fail to be swapped.

Frequently results in Spot the Imposter and Needle in a Stack of Needles. Compare Doppelganger Spin. See also Stolen MacGuffin Reveal.

Anime

 * D.Gray-man uses the person variation with Lenalee Lee.

Films -- Live-Action

 * Played with—interestingly enough—in Dragonball: Evolution. "Mai" disguises herself as "Chi-Chi", and in the ensuing fight, "Goku" mistakes the REAL "Chi-Chi" for the fake one, punching her and knocking her out. Whoops!
 * Johnny Five learns it from a street performer in Short Circuit 2.
 * Patriot Games. When a IRA terrorist is being transferred to another facility, several prisoner vans are sent out at the same time to confuse any possible attackers. It doesn't work because The Mole has told the terrorists which route the correct van is taking.
 * Home Alone 3 got a spy's shopping bag mixed up with an old woman's identical shopping bag.
 * What's Up, Doc? and the four red-plaid suitcases, containing: A) Barbra Streisand's underwear, B} Ryan O'Neal's igneous rocks, C) secret government documents, and D) a fortune in jewelry. Of course those would never get confused.
 * The a Team remake has a literal shell game played with shipping containers (moved by cranes), which would never work because, unlike a shell game, the crane never lets go of the crate.
 * In the French movie Oscar (with Louis de Funès) and its American remake (with Sylvester Stallone), several swappings of two identicial suitcases, one full of money and the other full of the maid's underwear, constitutes a main part of the plot.
 * Fritz Lang's film Spies features a Japanese diplomat giving three people identical envelopes to deliver, one of which supposedly contains a treaty. In reality, the diplomat has the actual treaty; all three envelopes are decoys.

Gamebooks

 * In book 2 of the Lone Wolf series, Fire on the Water, if short on money you can play a shell game in a tavern. If the hero masters either the Sixth Sense or Mind Over Matter disciplines, it's easy money as he has no trouble locating the marble through the cups.

Literature

 * Harry Potter
 * Harry Potter and The Philosopher's Stone has this happen with flying keys that all looked like.
 * In the beginning of Deathly Hallows, the Order has to transport Harry safely from the Dursleys' to the Burrow. They have six order members take Polyjuice potion with bits of Harry, turning them into exact replicas of him, so that when the Death Eaters attack they'll be unable to tell which is the real Harry.
 * An example of the actual shell game is found in Jingo, done by Vetinari of all people.
 * Good Omens has a version of this with babies. It doesn't go as planned.
 * In By Chaos Cursed, the last novel in Mickey Zucker Reichert's The Bifrost Guardians series, the medieval thief Taziar Medakan finds some hustlers in modern New York City running a shell game with playing cards. He takes offense at how much they cheat and sees it as his professional duty to cheat back even harder.
 * Not actually done in Going Postal but referred to as "Find the Lady" and how it's a game where a good con can cheat a man by making him think the con's a bad con but he's losing on purpose, y'see. It's a metaphore. Maybe five.

Live-Action TV

 * Star Trek: The Original Series episode "Whom Gods Destroy". Captain Garth ("LORD Garth!") Lord Garth shapechanges into the form of Captain Kirk and fights him, so neither the audience nor Spock know which one is which.
 * Also used in one of the movies. Kirk and a shapeshifter are fighting when the bad guys show up. They both claim the other is the real Kirk, and the shapeshifter gets blasted.
 * Used metaphorically in The Outer Limits: "O.B.I.T", for the reason why no department will admit responsibility for the mysterious machines of the episode title.
 * The B-plot of the Star Trek: Voyager episode "Live Fast and Prosper" involves Paris and Neelix trying to trick the Doctor with a literal shell game.
 * One of the oldest challenge in the Game Show Fort Boyard is a shell game (called "bonneteau" in French). Performed by Serge Avril, a stage magician, it consists of finding a little key under the cups; 3 keys (hence, 3 successes) in a limited time are necessary to win the challenge. Very, very few contestants ever made it through, 'cause the gambler is really good.
 * In a Kaamelott episode, a gambler playing the shell game easily swindles Karadoc out of all his gold. When he tries this with Perceval, however, the gambler soon discovers that The Fool just cannot be beaten at this game, a talent which the dismayed gambler dubs "the Eye of the Mole".
 * White Collar: Neal met Mozzie when Mozzie was running a street game of three-card monte with a partner. Neal used sleight of hand and an extra card hidden in his sleeve to cheat the game, and Mozzie was impressed enough to track Neal down and propose a partnership.
 * The pricing game with the same name in The Price Is Right. The game features four shells with one of them holding a winning ball. The contestant is asked whether each one of the four incorrectly price items is higher or lower. The contestant receives a chip for each correct guess and is an instant win if all four items are correctly guessed.
 * On Leverage this is the primary game of Nate's father. It is strongly contrasted with Nate's game of chess.
 * In Kaizoku Sentai Gokaiger: 199 Hero Great Battle, the Gokaigers and Goseigers trick Hades God Dragon into attacking his own Mooks by transforming the mooks into illusions of themselves.

Newspaper Comics

 * Beetle Bailey: Parodied in-universe by Sarge: Beetle is hiding in one of three large metal carbage cans, so Sarge shuffles them around quickly (he's just that strong) and pretends it's a shell game to teach him a lesson.

Video Games

 * In Paper Mario, out back of Koopa Village, you have to play a "shell game" where the prize is... Kooper's shell! The Fuzzy with the shell climbs the tree, then trades places with Fuzzies in other trees several times. Then you have to whack a tree. If it's the wrong tree, you'll have to fight a Fuzzy; if it's the right one, you go to the next round, where they switch around faster. Win that, and you move on to the final round, where they switch even faster. Win that, and you get the shell to give to Kooper.
 * Also, on Shiver Mountain and in Crystal Palace, there will be a few points where you have to use Kooper or Bombette, and once you do, there will be one or more Duplighosts mimicking the partner, and you have to decide which is the real one and which are the fakes. This involves watching what they say. (The Duplighosts with the "worst disguises ever" don't count.)
 * In Super Paper Mario, Mimi disguises herself as Merlee. One of these times, you're talking to the real Merlee, and they mix it up. Then you have to guess which one is the real one. You may ask 5 questions, so you can base this on Something Only They Would Say or Something They Would Never Say.
 * Harvest Moon FoMT has Won give the player a version of this. In this case, you need to guess which apple is which.
 * One Wario World boss requires you to find which cup he's hiding under.
 * There's a Mario Party game that has you play a shell game with a Koopa.
 * In the Dystopia map "Cybernetic", there is a Cyberspace objective in which you see a yellow box in a ring of about 20 blue boxes. The yellow box turns blue and they all spin around for a few seconds. Selecting the right box speeds the capture of the objective.
 * One of the between-match minigames in Mortal Kombat: Deadly Alliance (which returns in Mortal Kombat 9) is Test Your Sight, essentially a shell game. If the player succeeds repeatedly without making a mistake, the cups move faster, and eventually a fourth one is added. It's particularly funny when the player is using the blind swordsman Kenshi, since he tracks the movement of the cups by cupping one hand around his ear and listening to them.
 * Beyond Good and Evil: When first going to meet the Iris Network, Jade has to play a shell game (a very easy one) operated by Peepers (who may or may not be blind?) to find a key card for a room from where she may enter to the Iris' Den. At any time, the regular shell game (find the little pearl) can earn you good money, up to a limit. The more you bet, the quicker Peepers shuffles them.
 * In Pokémon Gold and Silver, Trickster Gym Leader Janine has all of her followers dress up as her so that you won't be able to tell who the real Janine is.
 * Prototype combines this with Spot the Impostor when you perform the "Patsy" move. Mercer, while in the form of a soldier, can grab another soldier, spin the both of them around, and point to the other guy to confuse the enemy into believing he's the impostor.
 * Batman: Arkham City had Riddler using his second hostage in one of these. In a twist, you are required to use Detective Vision, because he will lower the hostage through the floor and bring it up through another one.

Web Comics

 * Interestingly played with in Order of the Stick: Xykon (a lich) is attacking Azure City, but there appears to be three of them. Since this is a Stick Figure Comic, they're all walking skeletons that look identical to Xykon. Haley mentions the "shell game" aspect here. Which was, of course, the point. The player doesn't win a real shell game.
 * An interesting variant is referenced (and explained in the Footnote Fever) in Schlock Mercenary: the Turbingian Shell Game is a con involving at least two thieves. Thanks to the pickpocket running the con, one or more of your valuables will be hidden under one of three shells, along with an explosive bean. The shells are then smashed, one by one, with a large mallet. You have to try and guess which ones don't contain your valuables. While you are occupied doing that, the pickpocket's partner is removing everything else of value from your person.

Western Animation

 * Avatar: The Last Airbender: Toph finds out she can scam street magicians by playing this game. Because her earthbending Disability Superpower lets her sense where the pebble is, she can get it right every time. She even puts it back under a shell when a con artist tries to stealthily put it in his sleeve.
 * An episode of the Aladdin animated series has an ancient creature holding an item of mystic power that the group needs. He puts before them several tests that are borderline Ice Cream Koan versions of such things—such as "stick out your tongue and touch your forehead" (Aladdin sticks out his tongue... and touches his forehead with his finger). One of them is a shell game. The creature claims to have won after Aladdin chooses, but Aladdin has Abu and Carpet move the other two shells to reveal there was nothing under them... so clearly there must be something under the one he chose. (Invoking the "shell games aren't won by the player" point.)
 * In one episode of American Dad, a significant jar of peanut butter is swapped several times between two identical bags. Eventually one falls into lava, at which point Steve learns that Stan had been trying to do this, but had thought it was a "magic bag" and didn't realize he had to do anything to make the swap, resulting in the jar being in the bag that fell into lava.
 * In an episode of The Simpsons, Homer becomes a licensed Krusty impersonator and gets grabbed by The Mafiya when they mistake him for the real Krusty...who gets brought in at the same time. The mobsters are going to shoot Krusty and let Homer go, but Krusty grabs each of their heads and plays a human Shell Game, which succeeds in confusing the gangsters and winning them their freedom...until Homer says "Good one, Krusty!"

Real Life

 * The original of the phrase is from a form of gambling where the "dealer" hides a small item under a shell or cup or other device identical to two others, then quickly moves them so that whoever's watching can't tell which it was under. The player then picks one; if the item is under it, he wins. However, the point of a shell game is that with some quick sleight of hand, the "house" ensures the player wins the first round by putting an item under all three. In following games, a similar trick ensures the item isn't under any of them, and the dealer always wins.
 * The President of the United States when traveling by helicopter. Three or four identical helicopters are used so no outsider knows which one he is on. They also sometimes use several identical presidential limousines in motorcades.
 * In three-card monte, one of the real purposes of the game is to distract players so that a confederate in the audience can pickpocket them. Such a person is known as The Dip, or sometimes a "cutpurse" if you're in some sort of Epic Fantasy.