[Closi]

I think there is an edge case where both people die at the same time, but one is about to make a move and drops the piece as they die, and it lands on the board. Is that move valid? Or not as they are already dead when it lands?

[toast0]

Only the other player can challenge a move as illegal, right? If they're dead, they won't challenge and won't move, and thus lose?

[LorenPechtel]

Typically there will be a clock involved, your move is completed when you push the button on the clock. Merely dropping a piece on the board doesn't complete your move.

However, I can see a situation where the player dies and falls on the clock, pushing his button.