A units mismatch lost a Mars orbiter

In September 1999, the Mars Climate Orbiter went silent during orbit insertion and was never heard from again. One program's output files gave small thruster impulses in pound-force seconds instead of the specified newton-seconds, so navigation underestimated their effect by a factor of at least 4, adding up to a 169 km navigation error. NASA's lessons-learned record also blames process: the program was misclassified as non-mission-critical, and its interface with the navigation software was never tested.