That reminds me of a problem that cropped up on some of the earlier digital telephone exchanges with call forwarding.
Subscriber A set his line for calls to be forwarded to B, B forwarded to C, and C had calls forwarded to A. When somebody made a call to any one of those three numbers the system went crazy trying to forward the call indefinitely around the loop! If one or more of the subscribers was in a different CO, a whole trunk group could be busied out in a flash!
The software had been written to prevent B forwarding to A if A had calls forwarded to B, but apparently nobody had thought of the possibility of this happening with three or more numbers.
