Holding the door
Despite my comments on this site I've been brought up correctly... I hardly swear (unless something REALLY anoys me!), I'm polite, well-mannered and generally nice to people.
When I walk through a door and you're following me I'll hold the door open to make it easier for you to walk through. Likewise if I see you walking toward the door from the other side I'll push the door open and hold it for you. I do this because it's polite and friendly...
If you walk through the door and ignore the fact that I'm holding it for you the next time I see you I will make the point of shutting the door in your ugly fucking face you egotistical fucking prick!
It takes no time whatsoever to nod an acknowledgement, say "cheers!" or "Thanks" or even smile. You're rude to me and you'll get it back tenfold motherfucker!