the Mars Climate Orbiter
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The 1999 Mars probe lost because one team's software spoke pound-force seconds and the other expected newton seconds.
On September 23, 1999 the orbiter hit the Martian atmosphere far too low and was destroyed: ground software from Lockheed Martin reported thruster impulse in imperial pound-force seconds while JPL's navigation expected metric newton seconds, and months of tiny trajectory errors compounded silently. NASA's mishap board turned it into the definitive parable of interface contracts. Units are part of the type; the universe does not do implicit conversion.