This slip-up occurs in season two, episode twelve, the episode is called "The Piper".

Watch carefully when Carl is in the Piper's dressing room, sneaking around. He gets stuck in the Piper's trunk and Nick has to go save him. Nick goes into the room and free's Carl, only to close the trunk on him when the Piper comes through the door. When Nick takes off the Piper's costume head, he reveals an old tattered costume head, when he takes that off, he reveals the Piper's true face, a grey, ugly old Piper with chunks of hair missing off his head. He is mean and Nick points a gun at him, only to find out he is controlling children. Anyway, the Piper takes off running WITHOUT the costume head, remember Nick threw both heads on the floor. They turn a corner and all of a sudden, the Piper is wearing the costume head.

What gives? How did he get the costume head?