So, new question. In order to be a good DM, do you have to have them memorized?
MY answer is no. As a player, I try not to mention when my DM goes off on a rule (Barley ever, he seems to have a photografic memory, and when I do, I'm usually wrong). I just say that it is a house rule, which he seems to have a mound of now. The measure of a DM, is how good he makes the sesion. Not whether he was right about the rules, but that the game in itself was fun and enjoyed. And for that to happen, all of the players must overlook the rules sometimes.