It actually happens quite often but that's because I have long hair and wear skinny jeans (and half the time women's jeans even in drab) and it happens when I'm turned around.

But yesterday, I walk into a Subway and the sandwich "artist" says "What can I get you, ma'am?" Then I notice his eyes open wider as he realizes I'm no ma'am, while the girl working next to him tries hard to keep a straight face. He seemed to be too frozen to apologize, so to let him know it was no big deal I simply ask for my meal "This ma'am will have a... "

So the I was thinking... I must make a hideous woman while in drab! But, I figure it can be a good starting point for getting into girl mode