This slip-up is found in a number of movies. The scene is where Tap is getting on the airplane, where ultimately Derek Smalls, the bass player, takes a foil-wrapped cucumber out of his pants. Derek initially walks through the metal detector, and the metal detector goes off. He takes some keys and a tuning fork out of his pockets, puts them in the little box, then walks back through the detector. It doesn't go off. He turns around and walks through again, it goes off. Obviously, its a one-way metal detector.