Following the MCO failure, there was a lot of bashing for making a simple metric-English units conversion error. However, it wasn't until later that I found out the specific error that caused the mission failure. One of the parameters used in the trajectory calculations dealt with the delta-v imparted by unloading the reaction control wheels. This was a very small value that resulted in an error of perhaps 40 miles at the end of a flight several hundred million miles long. Unfortunately, the error was on the side where it resulted in hitting the atmosphere. Had the error resulted in the altitude being 40 miles too high, it wouldn't have been a problem. One trajectory parameter out of many resulted in an error that caused the mission failure. It was that subtle. I don't know why it wasn't caught by the tracking team but that's another story.