Yesterday I was standing in line with one item at Trader Joe's, and I was really sick so I was out of it.  There was only one person in line in the line ahead of me and beside me, and a lady with a ton of groceries got in line behind me.  A few seconds later she moved her cart to the side to the other line because it looked like they'd be done sooner, but she told me if they did finish before the people in front of me I should cut in front of her.  I thanked her, and then sort of lost track of what was going on since I was so sick.  Then the cashier in the line beside me said something to the effect that he was ready, ending with "miss", and I jumped and started to hand him me drink.  He looked slightly affronted and confused, and then I heard the lady behind me say "it's ok, he's going first. we had a pact."  I almost looked around for the guy she was talking about, and it took me a few moments to realize that when he said miss he was talking to her, and I was the guy that was going first.  I'm so used to not passing, and I haven't even been trying to pass for very long, that on top of being sick, I forgot that i'm a "he" not a "miss". 

Great feeling isnt it glad to hear the story.

I always like reading stuff like this. I don't really think I pass that much but apparently some people think differently. I noticed when I was traveling that a lot of people either don't usually use pronouns or weren't sure which one to use for me, so I hardly ever heard Miss or Sir, but on one plane when I was trying to find a place to put my carry-on in the over-head one of the people in my row said "Hey help this guy right here" and he made a spot for my bag.

Also one other funny thing, when I was waiting in line at customs, it was pretty long and I had some time to spare so I went to the bathroom but didn't even realize I had went into the men's toilet until I came back out and stood in the line some more and was looking around. No one said anything to me going in or coming out though lol.

There are those times when we pass and don't really realize that we are being taken as just another guy.  As you said you used the men's room and everyone considered you just another guy.  I think when we relax we can pass much easier.