And again, this isn't even because they've achieved what I'd consider system mastery.
What I'd call system mastery involves, to give a 3.5 example... being able to casually refer to an "oil of mending", an item that has never seen print, because you know that oils are the topical variant of potions in the rules, Mending is a cantrip in the rules that perfectly fixes an object with a single clean fracture, and those who can't cast spells would want to do that.
