Patriot missile failure at Dhahran (1991)

On 25 February 1991 a Patriot battery at Dhahran failed to engage an Iraqi Scud, which hit a barracks and killed 28 soldiers of the US Army's 14th Quartermaster Detachment. The system counted time in tenths of a second, and truncation in a 24-bit fixed-point register meant the error grew the longer it ran. After about 100 hours the clock had drifted by a third of a second, enough to shift the tracking window nearly 700 metres. The fault was known: Israeli data showing it reached the Army on 11 February, and a later warning said only that very long run times could affect targeting, without saying how long. It shows why long-running systems must be tested for how long they will actually run.