One of the things that really gets on my nerves is rude people who don't acknowledge it whatsoever when you hold the door open for them or something. Even a smile is good enough!

A week or so at work, some guy ordered a drink from me, and after making it for him he didn't have anywhere near enough money to pay for it. He started screwing at me because he couldn't have his drink, as if it was my fault he didn't have enough money. :[