That has got to be my all time biggest pet peeve.
Once I was waiting at a stoplight when this guy flicks his butt out the window and lands right at my feet in front of me. I rolled up to him and polity yet sarcastically said, "you accidentally dropped your cigarette." He replied, "No I am done with it." I then went off on him saying, "No you dropped it because I know you wouldn't litter. Or do you think that it is socially acceptable to throw trash on the ground? Why don't you throw your Coke can out there too?" I picked up his butt and threw it back in his car and that was with me holding back a ton. I wanted to punch his lights out after his comment.