In room 16506 there is a reappearing snake on the uppermost red trapdoor.  If Jack leaves the room after he has revealed the sinking brick below the lowermost red trapdoor (i.e. does not use that brick yet), then he has to open the trapdoors again and the snake will fall on the brick.  Jack will then have to commit suicide if he wants to ride down with the brick and hasn't got a shield.