From cardplayer.com

Allen Kessler stopped by to describe a situation that he had never before seen in a $10,000 championship event. Five players saw a flop with three diamonds, and everyone checked to the button, who bet. There was one caller, and everyone else folded. The turn card was a blank, and it again went check-bet-call. The river card was a fourth diamond, and this time the early position player bet out for $1,000, and the player on the button moved all in. The other player called immediately, saying, "I have the ace-high flush." Shocked, the player on the button said, "No, <i>I</i> have the ace-high flush. The early position player showed Ad-10d, and he flopped the nuts. The player on the button showed -- Ac-4d. He misread his hand! Unbelievably, he lost his entire $10,000 in starting chips on a hand that he should have folded on the flop.

Remember kids, there's no penalty for double-checking your cards!

Wow. What a gutwrenching feeling that must be.