A curtain card is a very, very bad thing. A curtain card is a little bit of card on which the hand is written (written in by the players if the cards are hand-dealt, printed if the cards are pre-dealt). I am not sure of their precise purpose, though when they are in use all players diligently check to make sure the cards in their hand match the cards written on the curtain-card. Basically, a curtain card is an accident waiting to happen. I'll also be the dummy. How is it ever possible that the result of a mistake on the curtain card would be that the board "could not be played"?