AT&T long-distance network collapse of 1990

For about nine hours on 15 January 1990, much of AT&T's long-distance network refused calls. A software update installed in December had a misplaced "break" in C code, which jumped out of a "switch" statement early in the routine that handled recovery messages. When one switch reset and announced "I'm back, I'm OK", its neighbours could crash while processing that message, then reset and announce themselves in turn. It is the classic cascading failure: the code meant to restore the network spread the fault across it, because every switch ran the same software. AT&T recovered by reverting to the earlier version, a reminder that a fast, tested rollback is part of any deployment.