The missing American Patriot Missile, 1991

In 1991, during the first Gulf War, an American Patriot Missile battery in Dharan, Saudi Arabia, failed to shoot down an incoming Iraqi Scud missile. The Scud missile hit an army barracks with many casualties. State-of-the-art computers controlled the Patriot missile, but there was a problem. To work Patriot needed to accurately know the time. This was done with an internal clock that started to tick when the computer was first switched on. However the computer program, when converting the internal clock time into the time used by the guidance system introduced a tiny mistake. It rounded the number down slightly. With each passing second the error became larger until finally it was enough to make the missile miss.

The moral: small mistakes in calculations often build into big mistakes.