What is the worst software bug in history?

Buffer overflows were only very rarely perpetrated by the gets() library function; since the mid-'90s, programs that called gets() would trigger a warning message on the console due to its insecurity. Far more vulnerabilities were caused by sprintf() than were ever caused by gets().

This would be a pedantic comment but for the insinuation that response made that buffer overflows in C/C++ had been somehow mitigated by changes to the C standard library in modern ANSI C. No.

If you include pre-history, I would say the one that causes people to see patterns that aren't really there.

http://en.wikipedia.org/wiki/Therac-25 ?

not quite 'the worst' but an interesting story no less - http://en.wikipedia.org/wiki/Siberian_pipeline_sabotage

Y2K