The Project Uncertainty Principle

Plans based on average assumptions are wrong on average.

—Sam Savage. The Flaw of Averages. 

There's a Dilbert cartoon from 2003 in which Dilbert is asked for a description of his project and its projected cost. He responds by declaring that, "The Project Uncertainty Principle says that if you understand a project, you won't know its cost and vice versa." I don't think Adams knew he was on to something.

You Can't Change Just One Thing

"You can't change just one thing" is an engineering principle that has a wide application beyond engineering. Applied to planning, it is a warning to consider secondary effects during the analysis phase.