"Anything that can go wrong will go wrong".
Some corollaries: (from Arthur Bloch: Murphy's Law and other reasons why things go wrong!)
Nothing is as easy as it looks.
Everything takes longer time 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.
Left to themselves, things tend to go from bad to worse.
Whenever you set out to do something, something else must be done first.
Every solution breeds new problems.
It is impossible to make anything foolproof because fools are so ingenious.
Nature always sides with the hidden flaw.
10 Versions of Murphy's Law for Universal 'Truths', ThoughtCo., 2018-12-31