Enter the maze

AT&T Crash, 1990

The day the phones stopped working...all of them.

In late 1989, AT&T engineers upgraded the software of their 114 US switching centres: the computers that make the connections so your phone links to the one you are calling. Each computer was duplicated so if one went wrong the other would take over. On January 15th 1990, they stopped working: 70 million calls failed. The problem was in a single line of code out of millions...and it was in both computers' copies. AT&T lost $1 billion as customers fled to their competitors.

The moral: With software, duplication doesn't always help.