If there is a possibility of several things going wrong, the one that will cause the most damage will be the first one to go wrong. If anything just cannot go wrong, it will anyway. If you perceive that there are four possible ways in which something can go wrong, and circumvent these, then a fifth way, unprepared for, will promptly develop. Left to themselves, things tend to go from bad to worse. If everything seems to be going well, you have obviously overlooked something.
Nature always sides with the hidden flaw.
Mother nature is a bitch. Murphy was an optimist.