I ran into somewhat of a similar situation last weekend.
Basically one player showed her hand, which was a flush. Her opponent mucked and she began to collect the pot. The board read K 6 2 9 6 with 3 hearts.
About 15 seconds later he said "damn I had 3 of a kind, kings. Oh wait, I had a full house". I believed him and the pot should have belonged to him, but he had already mucked and no one saw his hand. I awarded the pot to the player with the flush.