Murphy's Law: If anything can go wrong, it will.
That sounds rather grim, but many of the extensions and corollaries are worthy of a chuckle. I especially like this one because it reminds me of my days working as a computer programmer:
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.
Corollary: It will be impossible to fix the fifth fault, without breaking the fix on one or more of the others.
Murphy's Law on Gutters: When a gutter develops a large hole, the hole will appear directly over the most used door of the building.
Which is why we replaced 33 feet of gutter on the west side of the house today. Or, to be more precise, we watched two nice looking young men replace 33 feet of gutter on the west side of the house today.
Let it rain.