How would this be ruled in a tournament?
A player with the winning hand thinks he lost and is first to show his cards. He throws in his hand face up and the cards hit the muck. One of his cards, that doesn't play, ends up under the muck. Is that a dead hand?
Cards read when they're face up, regardless of the location (unless I'm missing something obvious). ... I know that once a face-down hand hits the muck, it's automatically a dead hand because the hand cannot be retrieved from the muck.
The cards must be "tabled". Meaning this player has to place his cards on the felt face up. Showing your hand isn't enough. If the card landed face down, in the muck, it is a dead hand. Even if that card didn't "play". The player must table two cards.