On December 11, 1998 the Mars Climate Orbiter was launched from Cape Canaveral, Florida. Ten months later, on September 23, 1999, NASA lost touch with the Climate Orbiter just as it was approaching the red planet. They never regained communication, and the $125 million spacecraft was considered a complete loss. The problem? Unit conversion.
Several teams had been working on the project. According to CNN, NASA had been working in metric units for several years. One of the teams working for a contractor, however, used English units (pounds, miles, inches, etc.), and the lack of conversion meant that the Climate Orbiter approached Mars from an altitude of 60 kilometers (37 miles) instead of 150 kilometers (93 miles).
The moral of the story: check your work, and pay attention to units.
(As a footnote, just the past January, NASA announced that it would use metric for all its operations related to the upcoming missions to the moon in order to make it easier to cooperate with other space agencies. Burma and Liberia are the only other countries that still primarily use English units instead of metric.)
Mars photo created by NASA and the European Space Agency