To use an analogy from cooking:

Mastery is when the chef not only knows how to make oatmeal the “right” way, but is able to answer if you ask “What happens if we use half the water? Twice as much butter? No butter at all? 4x the milk?”. Because they’ve tried all these things before.

A master understands where the canonical solution fits in the greater design space of all solutions, and the tradeoffs involved. They can explain, in depth, what happens if you “do it wrong”, and can provide examples of instances where you might want to.

A master understands when the laws of gravity actually break. ¹

1. My First Million Podcast Episode 52: https://www.mfmpod.com/99-years-of-charlie-munger-wisdom-in-44-minutes/