Mad Mother Psychoshower Curtain
From the site:
Either place this shower curtain facing away from the tub as a funny, decorative piece. Or, face it into the tub. When your "victim" takes a shower and pulls the curtain closed, they will get a frightening surprise. A hilarious prank to play on unsuspecting guests!
Of as an even more hilarious prank, actually stab them through the shower curtain with a knife! Ha ha ha! Oh the laughs to be had!