What happens if a player mistakenly thinks they have been checkmated? A quote from Wikipedia:


  The objective is to 'checkmate' the opponent's king by placing it under an inescapable threat of capture.


Suppose two players (A and B) play the game using the old fashioned way (no computer involved). If player A has player B's king checked from two positions.  Player B only knows how to block one of those two attacked positions. But there is a way for Player B to make a single move and block both attackers. What would be the proceeding of the game next? Should player A show player B how to do it? Or does the game end with player A winning even through this is not a checkmate?
If player B is checked in two different ways, then there is by definition no way for player B to block both attackers with a single move. The only way to get out of double check is to move the king.

If there's a square for player B's king to move to that is safe, then he can get out of the check. If player B simply doesn't see it, then in a friendly game player A can certainly show player B where to move his king. In a competitive game player A should probably just let player B lose on time or forfeit by making an illegal move.

If there's no square for player B's king to move to that is safe, then it is in fact checkmate. Player A wins.