The Fourth Law of Thermodynamics

Original Axiom:

Murphy's Law - "Anything that can go wrong will go wrong."

Corollarys:

Murphy's Extended Law - "If a series of events can go wrong, they will do so in the worst possible sequence."
Finagle's Law - "Anything that can go wrong, will—at the worst possible moment."
Hofstadter's Law - "It always takes longer than you expect, even when you take into account Hofstadter's Law."
Silverman's Paradox - "If Murphy's Law can go wrong, it will."
Flanagan's Precept - "Murphy and Finagle were incurable optimists."

And if you're not feeling cheered up yet, there's always the Murphy's Law Site.