Q: What rule or game mechanic are you CONSTANTLY having to look up?
No matter how many times it happens in your game, you still have to find the page every time.
A: Grappling. Ever single time someone grapples, is grappled, wants to attack into a grapple, what weapons you can use, how to escape etc. Whenever I hear the word grapple, I immediately reach for my PHB.
How about the rest of you?