Bill's Law

THEOREM: Strategy, Tactics and Logistics can be considered separately only in the realm of theory.

COROLLARY: Design, without regard to implementation, is flawed in the real world.

I've had these on my white boards for about 15 years. I do it to remind myself that you need to remind yourself that you have a REAL environment that you will play in, and not just some make believe world where everything behaves the way you want it to.