Abstract: A method and apparatus for tracing errors during operation in progress in a sequence including a transfer series of binary message words between a plurality of electric circuits, which form a new message word from the obtained message word for transfer to the next circuit. The first message word in a sequence is provided with a marking word, and in each circuit the marking word is added to the new message word formed on the basis of the received message, and each message word is sensed during the transfer, the message word containing a marking word being copied into a memory. All message words provided with a given marking word are subsequently read out from the memory to determine whether the sequence of message words provided with the marking word is correct.