People who don't read the menu properly or don't ask questions or ask for clarification. This is as much a part of my job as serving you your food. For example, Napolitana sauce has tomatoes in it and has done so for a very VERY long time. I for one won't make fun of anyone who doesn't know that. I will take the time to explain to you what is in a Nap Sauce and answer any questions you may have and suggest other options if you aren't sure. Just don't start breathing fire and demanding to speak to the manager if your pasta with Napolitana sauce has tomatoes in it and you didn't know.

That's it really, the rest are slightly annoying, I can pretty much take rudeness in my stride these days ( unless people are violent or aggressive) . Every job has it's bad bits, serving is no exception.