Duncan Smeed recently posted on his weblog about the 1996 explosion of an Ariane 5 rocket. The Ariane 5, a European rocket, self-destructed just 39 seconds after launch after onboard computers detected that the rocket was breaking up.
Coincidentally, the Ariane 5 rocket just had another odd failure, plunging straight into the Atlantic Ocean just three minutes after launch.
The cause of the 1996 disaster turned out to be a straighforward bug. Part of the software the Ariane 5 was running attempted to convert a 64-bit number into a 16-bit version, but the number was too large for the 16-bit register. The result was an overflow error which led to the shutdown of both the main guidance system and its backup (which was running identical software). The rocket then veered radically off-course and the self-destruct mechanism kicked in.
But the best part has to be that the calculation that led to the overflow error wasn’t even needed in-flight. According to an article about the incident by James Gleick,
One extra absurdity: the calculation containing the bug, which shut down the guidance system, which confused the on-board computer, which forced the rocket off course, actually served no purpose once the rocket was in the air. Its only function was to align the system before launch. So it should have been turned off. But engineers chose long ago, in an earlier version of the Ariane, to leave this function running for the first 40 seconds of flight — a “special feature” meant to make it easy to restart the system in the event of a brief hold in the countdown.
Source:
Sometimes a bug is more than just a nuisance,/a>. James Gleick, New York Times, December 1, 1996.