I had a similar exchange at a self-check-out lane a couple of weeks ago. The store has it set up so that one line feeds three automated check-out stations... the way you'd wait for a teller at a bank.
Anyway, I was at the front of the line with my daughter, there were maybe three or four people behind me when I noticed this couple pulling up alongside me. They were older, only had a few items and it was obvious that they weren't familiar with the procedure. Anyway, a station cleared and the lady shoves her cart in my path and starts complaining to her husband (in French) about what a bitch I was for butting in to her spot. I had been chatting with my daughter in English, so I guess she didn't think I understood what she was saying, until I turned to her and corrected her in French about there only being one line to serve all the stations and that she should have gone to the end of the line instead of trying to jump ahead of other customers. She got a bit huffy about being corrected, but took the next station and started ringing through her order. I noticed that she had all marked down items and couldn't figure out how to ring them through. I was finished my order so I stopped and showed her how to do it. She was very appreciative after all that, even though I'd been such a "bitch" for asserting myself in the line and correcting her erroneous assumptions about how the self-check-out line worked.