If there is a possibility of several things going wrong, the one that will cause the most damage will be the one to go wrong. If there is a worse time for something to go wrong, it will happen then. If anything simply cannot go wrong, it will anyway. If you perceive that there are four possible ways in which a procedure can go wrong, and circumvent these, then a fifth way, unprepared for, will promptly develop. If everything seems to be going well, you have obviously overlooked something.
Nature always sides with the hidden flaw.
It is impossible to make anything foolproof because fools are so ingenious. Whenever you set out to do something, something else must be done first. Every solution breeds new problems. Two wrongs are only the beginning. If at first you don't succeed, destroy all evidence that you tried.