THE SOFTWARE HORROR PICTURE SHOW
AT&T Long Distance Service Fail, 1990

On January 15, 1990, one of AT&T's switching-stations in Manhattan crashed. Station after station across America collapsed in a chain reaction, until fully half of AT&T's network collapsed. The collapse was caused by a software flaw in the front-end processors of all 114 of its 4ESS switching-stations.


Several Sources:

www-aix.gsi.de/~giese/swr/att2.html
catless.ncl.ac.uk/Risks/9.62.html
catless.ncl.ac.uk/Risks/9.63.html
catless.ncl.ac.uk/Risks/9.69.html
catless.ncl.ac.uk/Risks/9.70.html
www5.in.tum.de/~huckle/attcrash1.htm
stuff.mit.edu/hacker/part1.html