Some adages, laws & rules of thumb:

1. Murphy's Law
"If anything can go wrong, it will."
2. Pareto Principle
"80% of STH comes from 20% of STH else"
3. Parkinson's Law
"Work expands so as to fill the time available for its completion." or
"Data expands to fill the space available for storage"
4. SNAFU
"Situation Normal: All ****** Up" or
"things are in a mess — as usual"

More here. Quite a nice list.