I've faced something like this, and it was in a McDonalds as well. I was there with my family, and normally if I'm in public, I don't go to the bathroom, or I wait until I can find a unisex one. That wasn't an option this time, so I got up and went, and I just didn't feel confident enough to try going into the male restrooms, so I went into the female one. On the way in, I bumped into this girl, and I quickly mumbled an apology, and she looked at me weirdly and said "You do know this is the girls' room, right?" and I just panicked, and said "Yes". She didn't say anything else, and very quickly got out of there, and when I was in the stall, I burst out laughing. I tend to deal with these things by finding it funny at their confusion, and also it kind of cheered me up. This happens a lot, which I take as a compliment, since I seem to pass easily, which I never thought would happen, as I'm 4'11, and rather curvy.
I'm fairly confident with everything else, except bathrooms. I'll go there, look at the two doors, and I just can't do it. I don't know why.