NASA launched the Mars Climate Orbiter in 1998 to study Martian weather. One ground-software system supplied thruster data in English pound-force seconds while the navigation software expected metric newton-seconds. The accumulating error sent the spacecraft roughly 170 kilometers lower than planned, and contact disappeared on September 23, 1999. The metric system did not fail; the meeting did.
Sources: NASA Science · NASA Lessons Learned · NASA Jet Propulsion Laboratory
Image: NASA/JPL, public domain, via Wikimedia Commons.