AT&T outage of 1990 INVOLVES Software bug
The AT&T outage was caused by a bug in AT&T's own software, specifically a misplaced 'break' statement.
Explicit Confidence 99% · Jack Rhysider EP 169 Details
“AT&T admitted very publicly that the outage was caused by a bug in their own software.”
“One of their programmers put in the word ‘break’ in a wrong part of the code”