"NASA said on Wednesday it "made a mistake" in its calculations about a tear in space shuttle Atlantis' heat protection, but that it should not prevent the ship's safe return to earth.
Now when I say that I don't mean to belittle the importance of doing engineering calculations right the first time.
Yet it's unrealistic to expect a human being to be perfect. The only way to approach doing these types of calculations is with caution and a dispassionate disinterested manner that isn't rushed or affected by external forces. And then your work must be reviewed thoroughly over and over with a humble attitude until most of the bugs get worked out. The idea is to have a process in place to catch whatever human errors happen and to learn from the mistakes that are made.
Hopefully then the only errors left in a set of calculations are as small as the ones done by NASA above.