I feel better about it now, that I've got some time between the incident. It stuck with me the next morning and day until I got to my next event.
I have in my contract that I can paint approximately 12 faces per hour, so I believe she was aware that she was expecting a bit much. I did tell her that I could bring my quicker designs. I can't remember exactly what she was saying when she came to argue with me, but was telling me that the lady was saying she'd been waiting and no one said the line was closed. The father who was supposed to be last in line was in fact arguing my point with me, because the lady who hired me, didn't act like she believed what I was saying, so he kept piping up that I had told people several times that the line was closed.
While she was asking me to stay to paint these extra people, I said I was only being paid to paint til 8. She said everyone else stays. She said they ALWAYS stay.

I thought that was weird. I ended up calling the guy who did the event last year. I was curious if he'd had the same experience. He acted like to me that he didn't have any problems and that he was just really fast. But, when I asked him if he stayed later, he said no, that he never stays past the time he's hired.
The lady who hired me also told me that I should have had the guy who was last in line tell people behind him that the line was closed and then she told HIM that he should have been telling people behind him. I tried to tell her that you can ask customers to tell people behind them that the line was closed, but I've not had luck with people following through on that. I couldn't get that out though, because she only overtalked me. It looks like I need to get a sign, but do people really like wearing that thing?
After she left, the father waiting thanked me after I finished up with his daughter and told me I had a great spirit. That did lift me up a bit.