If there is a possibility of several things going wrong, the one that will go wrong first will be the one that will do the most damage. 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.
It is impossible to make anything foolproof, because fools are so ingenious. The consumer report on the item will come out a week after you've made your purchase. If you hit two keys on the typewriter, the one you don't want hits the paper. A conclusion is the place where you got tired of thinking. If everything is coming your way, you're in the wrong lane.