The Pareto Principle

I can't tell that it is very useful principle, but it nice to know:
Roughly 80% of the effects come from 20% of the causes

In testing it usually means that 20% of the bugs cause 80% of the crashes. In my practice it actually works.

Good more detailed article: Understanding the Pareto Principle (The 80/20 Rule). Surely read the last paragraph "Concluding Thoughts". The important thing in this principle - it is an observation, not a law of nature.