Anything that can go wrong will go wrong.
Nothing is as easy as it looks.
Everything takes longer than you think. If there is a possibility of several things going wrong, the one that will cause the most damage will be the one to go wrong. If anything simply cannot go wrong, it will anyway. If you perceive 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. Left to themselves, things tend to go from bad to worse.