Face-to-face, no screens. Cross-IMPs. The bidding:
South gingerly pulls out ONLY the 5♦ card from the bidding box and places it next to 1♦ card. South does not bother to put the Stop card on the table either. West passes without pause, North just waves a pass card (without putting it down) and East smoothly picks up his pass card from the table to "end" the auction.
East wins trick 1, and returns the suit without too much thought. A few tricks later, South claims 11 tricks.
East: "Plus 1"
East (to West): "Guess I could have saved the overtrick" -- Concurrently North (to East): "No overtrick, just made"
The next few sentences of dialogue reveals that East thought the contract was 4♦.
East calls the Director and claims damage, stating: (a) South did not use the Stop card and (b) East was fooled by the absence of a stack of bidding cards in front of South. East claims South's actions misled him to believe the contract was 4♦. East is a strong player and there's a good chance he'd have found the switch that defeats 5♦; he claims he didn't bother because of the partscore situation (3NT looked a possible contract for NS).
How do you rule?
PS: When this actually occurred, 5♦ was unbeatable. Although East was irritated after he discovered the contract was 5♦ and not 4♦, his side gained ~1 IMP (many tables were in 3NT making 10 or 11 tricks).