If there is a possibility of several things going wrong, the one that will do the most damage will go wrong first. Left to themselves, all things go from bad to worse. If you play with something long enough, you will surely break it. If everything seems to be going well, you have obviously overlooked something. If you see 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.
Nature always sides with the hidden flaw.
Mother Nature is a bitch. It is impossible to make anything foolproof, because fools are so ingenious. If a great deal of time has been expended seeking the answer to a problem with the only result being failure, the answer will be immediately obvious to the first unqualified person. If anything just cannot go wrong, it will anyway.