In theory, practice is the same as theory, but not in practice

Anyone who's tried their hand at research, built some software, or even run a kid's birthday party may relate to this idea: In theory, practice is the same as theory, but not in practice.

It's a lovely encapsulation of similar ideas to "All models are wrong, but some are useful" or "No plan survives contact with the enemy." Or one of my favourites, Hofstadter's Law: "It always takes longer than you expect, even when you account for Hofstadter's Law."

Also see: Physics envy

The quote, in varying forms, has been attributed to a number of people — QI suggests that Benjamin Brewster should get the credit from 1882.