It shits me when people fly up the left lane then try to merge in right at the end. I never let those people in.
Or, when it's a left lane must turn left situation, and people know that, but they zoom right down it, then just expect you to let them in, as legally, if their car is in front of you, they have right of way. Pfft, screw that. I just honk them and give them a gesture.
When I'm driving to Uni, and I let people in onto the main road, or let them turn off, it seems to make their day, they're so happy and wave thankyou's like 3 or 4 times. I always say thanks to someone who lets me in, so it suprises me there are so many arseholes out there who don't even give a slight wave or sign of appreciation.