Left to themselves, things tend to go from bad to worse.
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.
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. Corollary: If there is a worse time for something to go wrong, it will happen then.
If anything simply cannot go wrong, it will anyway.
Anything that can go wrong will go wrong.