The Grosvenor Gambit is a "psychological play, in which the opponent is purposely given the chance to gain one or more tricks, and often even to make the contract, but to do so he must play for his opponents to have acted illogically or incorrectly."
One pair, perhaps unintentionally, perpetrated one of these against me in yesterday's club game. I put a write-up with details of the card play on the Bridge Winners site.