4. Screwing up the flow of traffic by either not using a turn signal...
So I'm sitting over here in the left lane waiting to turn, and here you come with the right of way, and you're taking your sweet-ass time but I obviously have to wait for you to go by -- but wait, are you slowing down? Oh, you're turning. Information that would've been good to know 10 seconds ago.
5. ... or using it too early
By that same token, if I’m turning right, and you’re driving along with your blinker on, how am I supposed to know that you have no intention of turning? I see you with your turn signal, I check the other direction to make sure it’s clear, I look back at you, ready to go, and... WTF?