In any field of scientific endeavor, anything that can go wrong, will. If the possibility exists of several things going wrong, the one that will go wrong is the one that will do the most damage. Everything will go wrong at one time. 3.1 That time is always when you least expect it. If nothing can go wrong, something will.
Nothing is as easy as it looks.
Everything takes longer than you think. Left to themselves, things always go from bad to worse. Nature always sides with the hidden flaw. Given the most inappropriate time for something to go wrong, that's when it will occur. Mother Nature is a bitch.