Because if there's one thing HF21 should have told us: problems can always arise, but how we handle them, that's where the real value shows itself.

I think anyone that deals with coding knows this pretty well, you can be reviewing everything for days on end, just one little thing, one little thing that you didn't see or that you didn't think about can make the whole thing blow up... But no matter what, keeping calm and finding out what is the issue is the most important part.