Lost probe

Indeed, NASA lost an unmanned mission owing to a mix-up between metric and imperial units. In September 1999, its $125 million Mars Climate Orbiter probe was destroyed because its attitude-control system used imperial units but its navigation software used metric units. As a result, it was 100 kilometres too close to Mars when it tried to enter orbit around the planet. -Rey Ortiz

http://www.newscientist.com/article/dn17350-nasa-criticised-for-sticking-to-imperial-units.html