AT&T long-distance network collapse of 1990

On 15 January 1990 much of AT&T's long-distance telephone network in the United States stopped connecting calls for about nine hours. A software update to the network's 4ESS switches, installed in December 1989, contained a misplaced "break" statement in C recovery code. When a switch reset and announced it was back in service, neighbouring switches processing that message could fail and reset in turn, spreading the fault across the network. AT&T restored service by reverting the switches to the previous software version.

Text from OyList