Pet peeves, what gets your back up?

When there is no definitive line in check out, and people walk up to the counter and stand right next to you instead of behind you, in essence, ing in line. When the salesperson says "Can I help you?" to them, and they respond, I say, "Excuse me, but I was here before you were." and then they look at you as though you're a prima donna, instead of just asking for the rule of "first come, first served." Yep, I really, really hate that. All lines should be the way they are at Banks.