A player thinking he was the only player left in the hand. Throws his cards face up into the muck. Would those cards be dead?
In this case the hand is live and fine. Pick your rule book, the biggest two, and most end local rules end up being a variant of either of them, are the WSOP Live action rules or the TDA rules.
I think rule 106 best applies here:
- Cards thrown into the muck may be ruled dead. However, a hand that is clearly identifiable may be retrieved and ruled live at management’s discretion if doing so is in the best interest of the game. An extra effort will be made to rule a hand retrievable if it was folded as a result of dealer error or incorrect information given to/by a participant.
In your example case the player clearly had incorrect information, and could be easily construed in a way that he/she didn't see the other players hand that they had perhaps hidden or not made their hand clearly visible. Given their hand is face up it's easily retrievable. Given that it is also for the best interest of the game to not punish an honest mistake too it's likely again to be judged as live. Now of course the floor staff should be called and ultimately it is at their digression to determine the ruling but I would say the hand will likely be ruled as live.