I'll bet most of us know the wrong Murphy's Law. From murphyslaw.net :

Murphy's Original Law
If there are two or more ways to do something, and one of those ways can result in a catastrophe, then someone will do it.

Murphy's Law
If anything can go wrong -- it will.

Murphy's First Corollary
Left to themselves, things tend to go from bad to worse.

Murphy's Second Corollary
It is impossible to make anything foolproof because fools are so ingenious.

Quantized Revision of Murphy's Law
Everything goes wrong all at once.
Murphy's Constant

Matter will be damaged in direct proportion to its value.
The Murphy Philosophy

Smile... tomorrow will be worse.

Conclusions

If there is a possibility of several things going wrong, the one that will cause the most damage will be the first one to go wrong.

Corollary - If there is a worse time for something to go wrong, it will happen then.

If several things that could have gone wrong have not gone wrong, it would have been ultimately beneficial for them to have gone wrong.

Anything that can go wrong will go wrong.

If anything can't go wrong, it will anyway.

If you perceive that there are four possible ways in which something can go wrong, and circumvent these, then a fifth way, unprepared for, will promptly develop.

If everything seems to be going well, you have obviously overlooked something.

Everything takes longer than you think.

You never find a lost article until you replace it.

If nobody uses it, there's a reason.

You get the most of what you need the least.

Nature always sides with the hidden flaw.

Mother nature is a bitch.