Saw the situation happen to the guy who was supposed to be behind me in the self-checkout lane.
Guy: "You know, lady,there are things called lines and I was next!"
Cutter: "Who cares? I started checking my stuff already, pal. You snooze, you lose."
Man: grumbles under his breath and doesn't do anything, his face red from the furious wrath over the mere offense of getting cut in line.
I'm not ruining my day over an impolite person. Now, if I was in a hurry, I'd say something stating the timeliness of my situation, and politely awaiting a reply, for better or worse. It doesn't hurt to haggle, especially about your time. But a physical/violently verbal conflict over something like that is asinine in my opinion.