Patriot missile failure at Dhahran (1991)

On 25 February 1991, during the Gulf War, a Patriot air defence battery at Dhahran, Saudi Arabia, failed to intercept an incoming Iraqi Scud missile, which struck a US Army barracks and killed 28 soldiers of the 14th Quartermaster Detachment. The system's clock counted time in tenths of a second and stored the value in a 24-bit fixed-point register, so a small truncation error accumulated the longer the system ran. After about 100 hours of continuous operation the clock had drifted by about a third of a second, enough to shift the tracking window by roughly 600 metres so the Scud was not tracked. A software correction reached the battery the day after the attack.

Text from OyList