[from Goethe's Der Zauberlehrling via Paul Dukas's L'apprenti sorcier in the film Fantasia.] A bug in a protocol where, under some circumstances, the receipt of a message causes multiple messages to be sent, each of which, when received, triggers the same bug. Used esp. of such behavior caused by bounce message loops in email software. Compare broadcast storm, network meltdown, software laser, ARMM.