You may have heard of “Murphy’s Law” before (see item #1 below), but did you know that there are at least four five different formulations of this famous law? Via Roger’s Bacon, here are the first four:
1. Murphy’s Law – “Anything that can go wrong will go wrong.”
2. Murphy’s Second Law – “Nothing is as easy as it looks.”
3. Murphy’s Third Law (also known as Hofstadter’s law) – “Everything takes longer than you think it will (even when you account for Murphy’s Third Law).”
4. Murphy’s Fourth Law – “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.)
Bonus: Murphy’s Fifth Law (hat tip: Sheree) — “It will cost twice as much as anticipated.”
prior probability 
You’ve forgotten the fifth one: it will cost twice as much as anticipated.
I just added it to the list — that’s the most important one!
Absolutely