Laws, Theorems, Principles, etc.

Betteridge's Law of Headlines
Any headline which ends in a question mark can be answered by the word 'no'.
Brooks' Law
Adding manpower to a late software project makes it later.
Clarke's Third Law
Any sufficiently advanced technology is indistinguishable from magic.
Cunningham's Law
The best way to get the right answer on the Internet is not to ask a question, it’s to post the wrong answer.
Doctorow's Law
Anytime someone puts a lock on something you own, against your wishes, and doesn't give you the key, they're not doing it for your benefit.
Finagle's Law (related to Murphy's Law)
Anything that can go wrong, will – at the worst possible time.
Gall's Law
A complex system that works is invariably found to have evolved from a simple system that worked.
Godwin's Law
As an online discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches one.
Hanlon's Razor
Never attribute to malice that which can be adequately explained by stupidity.
Herblock's Law
If it's good, they'll stop making it.
Hofstadter's Law
It always takes longer than you expect, even when you take into account Hofstadter's Law.
Murphy's Law
Anything that can go wrong will go wrong.
Occam's Razor
Entia non sunt multiplicanda praeter necessitatem. (Explanations should never multiply causes without necessity.) When two or more explanations are offered for a phenomenon, the simplest full explanation is preferable.
Pareto Principle
For many phenomena 80% of consequences stem from 20% of the causes.
Parkinson's Law
Work expands to fill the time available for its completion.
Parkinson's Law of Triviality
The time spent on any agenda item will be in inverse proportion to the sum of money involved.
Peter Principle
Managers rise to their level of incompetence.
Shirky Principle
Institutions will try to preserve the problem to which they are the solution.
Streisand Effect
The phenomenon whereby an attempt to hide, remove, or censor a piece of information has the unintended consequence of publicizing the information more widely, usually facilitated by the Internet.
Sturgeon's Law (Sturgeon's Revelation)
Ninety percent of everything is crap.
Zawinski's Law
Every program attempts to expand until it can read mail. Those programs which cannot expand are replaced by ones which can.