They screwed up big time

And another because of a sign error.

Then there was the Mars Climate Observer. In that case it wasn't the NASA engineers' fault, but Lockheed-Martin. The computers NASA used were expecting figures in metric, which makes sense because that's the system that the entire scientific community uses. Lockheed-Martin, however, made computers that gave their readings in imperial, causing the orbiter to hit the atmosphere of Mars way too low and break up due to friction with the air.