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How a Simple Bug in C Code Crippled AT&T’s Network: The 1990 Switch Statement Catastrophe and Lessons for Software Engineers
Introduction
On January 15, 1990, AT&T suffered one of the most severe network outages in U.S. telecommunications history. For a period of nine hours, roughly 60,000 customers-the backbone of America’s communication system-found themselves unable to make long-distance phone calls. Businesses stalled, families struggled to connect, and emergency services were impaired. The culprit? Not a malicious hacker or a massive hardware breakdown, but a tiny, nearly invisible flaw in a software switch statement written in the C programming language.
This incident didn’t just baffle technicians and inconvenience users; it rang alarm bells across the tech industry, signaling with crystal clarity just how potent and wide-reaching the effects of minor programming errors can be in large, interconnected systems. In this detailed case study, we unravel exactly how a single missing break in a C switch statement toppled a vital slice of national infrastructure-and, more importantly, what developers and organizations can learn from this disaster.
