IIRC, 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.
The interesting thing is that Uncle Sam has decreed (in a 1988 Act of Legislation), that all U.S. Govt Depts, including NASA, go all-metric.
The Inspector General has been hassling and pressuring all Depts to implement this change, under the enacted legislation.
Unfortunately, NASA, like so many other Depts, Entities, & Divisions... can't swap over to metrics overnight. Virtually everything they currently have, and operate with, is rooted in Imperial measure design. The 30 year old Space Shuttle design, is all-imperial measurements.
NASA have estinated the cost to convert fully to SI measurements is around $370M - almost half the cost of a Shuttle launch. Then, there's still the hangover of all the items that were built in Imperial sizes, that will still be around for a while yet. The unspoken thing is, that conversion to a full SI system, still isn't going to eliminate another possible disaster due to measurement system confusion.