Kerckhoffs's principle

Link: http://en.wikipedia.org/wiki/Kerckhoffs's_principle

A cryptosystem should be secure even if everything about the system, except the key, is public knowledge.
Sames goes for security.

See also: Security through obscurity (avoid going this way if possible).

Principle of least surprise

AKA principle of least astonishment, or PLOS / PLOA / PLA.

Parkinson's Law of Triviality

Link: http://en.wikipedia.org/wiki/Parkinson's_Law_of_Triviality

Dunning-Kruger effect

Link: http://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect

Murphy's law

Link: http://en.wikipedia.org/wiki/Murphy's_law

Anything that can go wrong, will go wrong