Mock Guffin

Our heroes have found a golden helmet, a powerful relic, or magic sword, something of great value, they take it to get appraised only to discover that its actually... junk.

A hapless man finds a briefcase, locked, in the back of his car. Perhaps it is full of money, secret documents or contraband. When he opens it he discovers... it's full of shredded paper.

An object of absolutely no significance that the protagonist mistakes, either through misunderstanding or excited imagining, as something of great importance, be it a magic sword or mysterious briefcase. Its importance to the overall plot is usually negligible, though an entire side plot can crop up because of it. All it turns out to be is completely worthless, and not plot-relevant like a real MacGuffin would be.

Compare with It's All Junk, Worthless Yellow Rocks. Not to be confused with All That Glitters or It's the Journey That Counts, which are often materially worthless but at least impart a valuable lesson. See A MacGuffin Full of Money for a plot device that can end in this.

Its opposite is Grail in the Garbage, where a seemingly worthless object is in fact worth a king's ransom.

Comic Books

 * One of the recurring gags in Knights of the Dinner Table is the players mistaking some piece of random dungeon dressing for a powerful magical artifact. The longest running of these jokes was Dave's 'magical' cow Chelsie.
 * And thanks to their Bag of Holding having a nigh unlimited capacity, they would clean out everything in a dungeon including toe nail clippings.
 * One Calvin and Hobbes arc had Calvin trying to find dinosaur bones in his yard. He ended up digging up a bunch of random trash, but until his mother pointed it out, he had no idea that there was something fishy about the "Calvinosaur" having a bottle for a skull, tin cans for a spine, and forks for arms.
 * X-Men Noir: Mark of Cain, reimagines the titular mutant superheroes as a gang of hired mercs looking for the fabled gem of Cytorrak. It eventually turns out to be a worthless fake made of glass. However,.

Film

 * The 'Holy Grail' in the Robin Williams film The Fisher King is most likely nothing, but his insanity has convinced him it's the true grail.
 * Burn After Reading is a rare case of of a plot that does center around this: Brad Pitt's character mistakes a CD of a low-level CIA analyst's mundane and unclassified memoirs for something top-secret. The analyst is also convinced that it's a vitally important, but only because he's delusional enough to think it has the makings of a "Washington tell-all bestseller."
 * The Coens also used this trope in The Big Lebowski (Walter's briefcase full of dirty undies, Lebowski's briefcase full of phonebooks.)
 * The Maltese Falcon, anyone?
 * Something of a subversion, as the fake one in the movie was substituted by the legitimate owner to prevent theft of the real one.
 * That's what Gutman thinks, but really there's no evidence either way. Maybe the legitimate owner always had a fake. Maybe there never was a real one. (Apparently, no-one has ever tried to scratch off the black enamel to see if it's really gold underneath before.)
 * In what's perhaps a subversion of the subversion a sequel "The Black Bird" has Sam Spade Jr. (George Segal) getting involved with a new group of motley villains looking for the statue that his father kept all these years.
 * Raiders of the Lost Ark first plays with this trope and then subverts it in the last scene.
 * In Dartagnans Daughter, much of the plot is driving by characters misinterpreting a laundry list and a really bad love poem as secret coded messages, and acting upon what they think these 'messages' are telling them to do.
 * In Flushed Away, both Rita and Toad fight for possession of what they think is a priceless gem. Roddy recognizes it as a cheap glass bead and proves it by smashing it to bits, which doesn't sit well with Rita.
 * In part of National Treasure, Ben Gates buys two copies of the Declaration of Independence at the gift shop(one of them is the real thing that the clerk believes to be a replica, the other a replica). They both become useful; when Ian and his cronies are trying to steal the real one from Gates, he throws them the fake to buy some time.

Literature

 * The (former) Trope Namer is the Golden Helmet of Mambrino from Don Quixote. (It even gets a song dedicated to it in Man of La Mancha.)
 * In the Star Wars Expanded Universe, the novel Millennium Falcon is an extended homage to The Maltese Falcon. Guess what the "Lost Treasure of the Old Republic" turns out to be. For that matter, a similar story was written thirty years ago, Han Solo and the Lost Legacy.
 * Actually, in the case of Millennium Falcon, it's even more annoying than usual. This "lost treasure" was supposedly left in the dying days of the Republic as something that would critically undermine Palpatine. Once the treasure is revealed, it pretty clearly demonstrates how inept and impotent Palpatine's opposition must've been.
 * After a life-threatening quest to recover, Harry Potter is devastated to realise . Also inverted:
 * In the Vorkosigan Saga story The Vor Game, Miles finds a body wedged in a drain pipe, who had drowned when the pipe filled up during a storm. Investigating why the man was in the pipe in the first place, he figures out that the dead man had intended to retrieve a package he had stored in a different pipe, only he had entered the wrong pipe, got stuck, and drowned. Miles finds the package and opens it, revealing... some home-baked cookies that the dead soldier had been hiding from his barracks-mates.
 * The Mouse That Roared is the story of a tiny nation that accidentally acquires the prototype "Q-Bomb," a weapon with planet destroying capabilities. They use it to hold the world hostage. At the end, the weapon's designer realizes that it was a dud.

Live Action TV

 * Gilligans Island once had a plot where the castaways find a suitcase which, at a glance, appears to contain top-secret spy documents, leaving them in danger of being chased by enemy spies. One Dream Sequence later, the documents fall out and they turn out to be spy documents from the second World War, which had ended twenty years prior and nobody was after.
 * While Antiques Roadshow mostly shows the inverse, occasionally a guest has something he believes to be a priceless artifact that turns out to be a cheap knockoff or common as dirt.
 * Similarly, many Mock Guffins come through Rick's door on Pawn Stars.
 * In the fifth season episode of Angel entitled "Destiny," Angel and Spike beat the living crud out of each other in their race to drink from the Cup of Eternal Torment. If my memory serves, the cup turned out to have a "Made in China" sticker on the bottom and was filled with Mountain Dew.
 * On Buffy the Vampire Slayer Spike and Harmony are searching for the Gem of Amara, a relic that makes vampires invulnerable. They find it in a tomb full of jewelry and other artifacts. Spike thinks it's a huge gaudy necklace and puts it on. Moments later he tries to kill Harmony in a fit of rage only to discover when he fails that the real Gem was a relatively nondescript ring that Harmony had put on.
 * In SeaQuest DSV, Krieg discovers a cache of glowing gems on the ocean floor and hoards as many as he can aboard ship...only to discover that they're bioluminescent fish poop, which trades its glow for an unbearable smell as it melts.
 * An episode of Bones has the eponymous scientist thinking she's stumbled across this. A prop sword is treated as much more valuable than its component parts simply because the movie it appeared in is so popular. As with much of anything, Bones does not understand why it is so. Probably because people seem willing to kill for the damned thing.
 * The movie was not so much popular as known to be the first Arthurian film ever made (or one of the first). This made the sword valuable as a part of film history.

Video Games
"Wess: As for what this "certain important item" is... No... There is no need for me to tell you. If a thief can't determine the value of what he steals, he's a disgrace to the profession."
 * In Mother 3, Wess sends Duster into a castle to retrieve a magical artifact critical for the safety of the world. Unfortunately for him, he's not told specifically what it is. He finds an interesting urn of some kind and brings it back to Master Wess. It turns out to be something else completely useless (the Noble Spittoon) and Wess is furious.
 * For the record, it WAS Wess's own fault for not mentioning that the item was magical, protected by a doorway that can only be opened by a special dance Duster was taught in his childhood, or anything other than the thing Duster was supposed to get was shiny.


 * There are fake Master Swords all over the place in The Legend of Zelda: A Link to the Past. The game even starts to remark on how amazing it is that you retrieved it for a moment before revealing the fake. You're not likely to be fooled, though, especially after seeing what the real Master Sword looks like.
 * In Nethack, there are fake Amulets of Yendor that, if you try to use at the end game, will cause you not only to lose the game, but have the gods mock you.

Web Comic

 * Eight Bit Theater had the armoire of invincibility, which is just regular old heavy furniture.
 * Technically, the armoire itself is invincible, in that it cannot be destroyed. However, the invincibility does not extend beyond the armoire, so it's pretty worthless.
 * Also, the bottom is made of cheap cardboard.
 * And it's not the artifact Fighter was looking for in the first place. Matoya had the Armoire and Armor of Invincibility, and gave Fighter the wrong one.
 * In The KAMics Gertrude & Brunhilda thought they had found something valuable, then were told otherwise.

Western Animation

 * A few episodes of The Simpsons have involved a Golden Helmet at one point or another.
 * In an episode of Futurama that parodies the film Titanic, Bender's love interest gets sucked into a black hole, leaving him with only her bracelet (a parody of "The Heart of the Sea") to console the saddened robot (with its monetary value). Upon request, Hermes promptly inspects it and informs Bender that "[i]t's fake, mon," sending him into even greater despair(about its monetary value).
 * Stroker and Hoop plays it straight, and then subverts it in an episode. A two-piece, ancient, mystical Chinese sword, wanted by ninjas, when put together, turns out to just light up like a flashlight, which Stroker notes was probably mind-blowing in ancient China.
 * American Dad had an episode where Steve and Roger are tasked with finding an old man's inheritance to his son. It's just a burlap bag that light's up when opened.

Real Life
"There was nothing in Al Capone's vault, But it wasn't Geraldo's fault!"
 * In the mid '80s, a secret vault was found in a building once owned by Al Capone. Geraldo Rivera hosted a prime time special devoted to opening the vault and finding what treasures it contained. Anything could be in there. Money! Bodies! Pictures of J. Edgar Hoover in compromising situations. Anything at all. Unfortunately, "anything" turned out to be "nothing". On live TV. After two hours of build up. Awk-ward!
 * Homer Simpson even started to write a song about it before he came up with the B-Sharps' hit single Baby On Board:


 * A news article reported about a mugger who stole a plastic bag from an old woman walking her dog. The bag, of course, turned out to be full of dog shit.
 * There is a very old form of scam (so old it's incredible people continue to fall for it today) where an apparently mentally challenged man approaches a victim in the street offering her (as it is often an old woman) the chance to buy some "stamps" in an envelope or suitcase. The "stamps" are bills of the highest denomination, so the full content can easily sum thousands or even millions, but the supposed retard, apparently unaware of the value of bills, asks only for some hundreds in coins. The victim is then approached by another "smart" guy that offers her to buy the "stamps" together and divide them between themselves. The victim agrees, and the smart guy always gets the upper half of the money while she gets the other which, as she realizes when arriving home, it's just sheets of newspaper.
 * In Spain that scam is not only old, it's actually famous, to the point that: a) it has its own name ("el timo de la estampita"), b) said name has became shorthand to indicate that something looks suspiciously scammy, and c) it was acted out in a 60s comedy film (Los Tramposos) as a plot point, with Tony Leblanc as the "retard" and Antonio Ozores as the "smart". The whole scene is here. Of course, with so much exposure, some people still fall for it. It's one of the quintessential scams also because, like all good scams, preys upon the unsavoury characteristics of the victim (greed and unscrupulousness) in a way that if you fell for it you'd be deeply ashamed to confess it.