If you perceive that there are four possible way in which a procedure can go wrong and circumvent these, then a fifth way, unprepared for, will promptly develop. It is impossible to make anything foolproof because fools are so ingenious. If everything seems to be going well, you have obviously overlooked something. Left to themselves, things tend to go from bad to worse. Nature always sides with the hidden flaw.
Mother nature is a bitch.
Whenever you set out to do something, something else must be done first. Every solution breeds new problems. Things get worse under pressure. Everything goes wrong all at once. The chance of the bread falling with the buttered side down is directly proportional to the cost of the carpet.