As we have the ability to create a wiki, it would seem relevant to produce an authoritative list of project management maxims, adages and laws.
Project Management Stack Exchange is a question and answer site for project managers. It only takes a minute to sign up.Sign up to join this community
Amara's Law: We overestimate the benefits of technology in the short run and underestimate its benefits in the long run.
Dilbert Principle: The most ineffective workers are systematically moved to the place where they can do the least damage: management.
Dunning-Kruger: Unskilled workers suffer from illusory superiority.
Finagle's Law: What can go wrong will go wrong...at the worst possible time.
Hofstadter's Law: It will always take longer than expected, even if you take Hofstadter's law into account.
Muphrey's Law: When you criticize someone's writing, the criticism will have a grammar or spelling flaw.
Occam's Razor: With multiple alternative explanations, the simplest one is usually correct.
Pareto's Principle: 80% of the consequences stem from 20% of the causes.
Segal's Law: A man with one watch knows what time it is; a man with two is never sure.
Work expands to fill available time
This is such an important rule to understand when you're scheduling. The moment you say 'this is going to take a week', it suddenly takes a week because people think, 'well, I've got a week so I'll just add this thing' or 'well, I've got a week so I'll just kick back for a day or two'. It's quite closely related to the idea of gold plating.
Whatever can go wrong, will go wrong
Basically, when you plan for risk this should always be your starting point. Not necessarily to think that things will go wrong but that even edge cases can happen in real life.
A related adage might be Donald Rumsfelds' famous quote:
There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know.
When you ignore the horrible English, it actually makes some sense - there are some unlikely risks you can predict (the known unknowns), and some that you can't (the unknown unknowns).