What are the rules (in your mind) regarding handicap accessible stalls in public restrooms?

Here is a story posted on another board I frequent:

“While out and about at the movies, there was a line to use the ladies restroom. I was second in line and there was a non-wheelchair user occupying the accessible stall. When she walked out of it, without saying a word, the woman in front of me began to walk forward to use it. Since I am on wheels, I immediately stopped her and said, ‘excuse me, but I need that restroom!’ She responded with ‘I’m sorry but I have to go and I was next in line.’ She proceeds to go ahead but I was able to roll faster AND beat her. She began to complain that her having to go badly constituted an emergency, and therefore since she was next in line, she should have been able to use the bathroom. I kindly told her that there was nothing she could do about it and that other stalls would be free soon.”

It got me thinking about this. Did the next woman in line have every right to take that stall and make the other woman wait her turn? Should she have let the other woman go in front of her and waited for the next regular stall? Personally I think the woman in the chair was out of line by rolling right past her into the stall and speaking to her so rudely.

What rules do you follow when using a public restroom? Are there other situations (invisible disabilities, claustrophobia, obesity, a mom with small kids), that should give seemingly able bodied folks equal access to these stalls?