While not exactly a question, you would be surprised by how often I hear this.

No. It is impossible to accidentally buy something by sneezing our scratching your ear. 1st., To bid at an auction, the bidder must register with the auction company in advance and receive a bidders number. All purchases are confirmed by this bidders number.

Simply, if you are not registered, your bid is not accepted or binding.

Yes, occasionally regular bidders known to the auctioneer may use a bidding "signal" or have an absentee bid that allows the auctioneer to bid on their behalf to a limit.

Professional auctioneers selling items of true value do not take bids from those who have not pre-registered.

While it makes a great urban legend, it simply doesn't happen.