Accidental Bid

Because of innocent or unrelated behavior during a silent auction, a character finds he has unintentionally bid an outlandish price on—and won—some expensive item. The accidental bidder will usually accept the purchase because he's too embarrassed to explain that it was a misunderstanding. If he does try to explain that it was a misunderstanding, the auctioneer won't care anyway, and he'll force the bidder to accept the purchase, if necessarily by use of violence.

Usually a Comedy Trope, but sometimes used as plot device leading to an adventure of sorts; The more useless an auction item looks, the more likely it is to be either magical or contain mysterious old maps.

Examples of Accidental Bid include:

Advertising

A "Head and Shoulders" ad showed a man accidentally buying an expensive icon because he kept scratching his head with his bidding number in his hand.
An ad for Mikado chocolate biscuit sticks has a man in the middle of an auction pulling a Mikado out of its box, and this being mistaken as a bid for a stuffed yak. It ends with him guiltily eating the biscuit while his wife glares at him.

Anime and Manga

Hunter X Hunter. Gon's allies decide to teach him how auctions work -after- it starts. Gon 'bids' an insane amount of money. Fortunately they are outbid and escape the consequences of his foolishness.

Comic Books

Was a staple device of children's comics like The Beano back in the 1990s (and may still be).
Donald accidentally bought an old boat in an auction in a Carl Barks story.
Goofy accidentally bought a painting while trying to tell the hours to a deaf man.
- This has been a staple device of Disney comics ever since Carl Barks. Goofy and the ducks are especially prone to it.
Hagar the Horrible, or actually, Lucky Eddie, did this once when he was waving to someone in an auction. To make the whole thing even more embarrassing, it was at a harem girls auction in the middle east.
Used at least once in an Archie comic: Betty tries to tell Archie - over traffic noise - how much money she's saved up and ends up accidentally getting the winning bid on a stuffed moose's head. Considering how often staple comedy devices are used in Archie Comics, this is probably not the only example in existence.

Film

In Lara Croft: Tomb Raider, Lara attends an auction to meet with someone and accidentally bids on something expensive and fairly hideous. Her face betrays her relief when she is outbid.
In Mousehunt, Lars and Ernie Smuntz inherited their father's house and tried to sell it in an auction. While they were chasing a mouse, Ernie accidentally made a bid but quickly undid the misunderstanding.

Literature

Paddington Bear once went to an auction. He found it a very friendly place. People kept on waving at him, so naturally he waved back...
In The Bagthorpe Saga the father chose the worst possible bidding sign for someone who'd spent the night before staying up hoping to see ghosts—a half-stifled yawn. Predictably, he ended up buying a large amount of useless items, including an ancient gramophone that plagued them for quite some time. Due to a lack of coordination, he also accidentally got into a bidding war with his wife over several pieces of bad furniture they wanted for the house.
In the children's novel By The Great Horn Spoon, the characters are talking among themselves during an auction. At one point Jack says the word "ate" in response to another character's question, and the auctioneer overhears and interprets this as an eight-dollar bid for what turns out to be a bushel of neckties (for comparison, the last genuine bid was at two dollars).

Live-Action TV

May have originated on The Dick Van Dyke Show, in an episode where the characters (who are TV show writers) make accidental bids while joking about writing a sketch where the character makes accidental bids.
Perfect Strangers did this with a bottle of wine, by having Larry try to teach Balki the basics of how these auctions work and end up winning the wine... and then having Balki tickle some bidders into making Accidental Bids of their own to make a profit from the wine.
Sanford and Son had Fred perform outlandish gesticulations to prompt the other bidders to go higher—after seeing Lamont do it—and he ended up buying the very piece the two had put up for sale, at a ridiculous markup.
The Golden Girls, episode "The Auction".
On Friends, Joey once bought an expensive boat at auction because he thought that he was meant to guess the price of the boat rather than pay that amount.
In Arrested Development, Buster accidentally bids on his mother's best friend and chief social rival, whom he was trying to avoid, when he was supposed to bid on his mother.
Subverted in an episode of Forever Knight. Nick really is bidding on the item, but his partner Schanke, who doesn't yet know that Nick is insanely wealthy, thinks he's doing it by accident.
In an episode of Psych, Gus sniffs around people at an auction, looking for a certain perfume that Shawn deduced the murderer was wearing. The auctioneer thinks Gus is bidding, so Gus ends up bidding on and winning... an old and quite large Confederate flag. Gus is black.
Discussed in an episode of No Reservations—in the Korean episode, Tony notices that everyone's bidding on fish by raising their hands madly. He tells Nari to keep her hand down or she'll end up buying a ton of cod.
Didi Mocó accidentally bought several stuff while talking to other people in an auction.
Happened to Dwight on The Office. He thought the other bidders were guessing the value of the items wrong and takes great care to put in the correct amount. He ends up getting every item on the auction, then realizes that he has to actually buy them.

Western Animation

The Real Ghostbusters had Peter Venkman win a book containing the Horsemen of the Apocalypse by sneezing during an auction.
Happened in an episode of the 1980s cartoon series Alvin and The Chipmunks, where Alvin was pretending to bid on expensive things in order to impress a rich girl; he ended up accidentally outbidding his rival on a yacht.
Something like this happens on The Simpsons: Bart wins an auction, and the auctioneer announces that he has win the item in question—at which he point he sniggers and runs away. The auctioneer, mildly annoyed, announces that the item goes to the next-highest bid—at which point a man in a suit sniggers and runs away in an identical fashion. The auctioneer ruefully inquires if anyone made a genuine bid—the entire room sniggers and runs away.
Inverted on Family Guy; Stewie wants the doomsday weapon, but can't get the auctioneer's attention, being only a foot tall. Eventually, the price falls to 'free' and nobody is seen bidding.
Done with a sneeze on the Totally Spies! episode "Do You Believe In Magic?". Sam then laments that the mission has gone over-budget.
In a Hurricanes episode, Napper Thompson accidentally made a winning bid for the McGuffin while knocking down a man who was really trying to buy it. Even worse: the man was representing the Big Bad, Stavros Garkos. Later on, Garkos bought it from Napper, who never really wanted the McGuffin in the first place, in another auction, where one of Garkos' thugs accidentally bought another item.

Real Life

A bit of Truth in Television here: The emperor Caligula did this to the senator Aponius Saturninus when he fell asleep at an auction. He told the auctioneer to pay attention to the senator who kept nodding at him. Aponius wound up buying thirteen gladiators for a large chunk of change.