If you could enforce a rule of etiquette, what would it be?

We all have things that annoy us, chap our fannies, or just plain piss us off. One of mine is people who interrupt while you are having a conversation. What’s yours? And we do them to others, too. Do you chew with your mouth open? Are you one of those who combs their hair in the kitchen? I know, I bet you leave the toilet seat up in the middle of the night, don’t you? Bastards, the lot of you. So tell me, what yanks YOUR crank?