This was inspired by the "do you look younger than you are" thread. I was reflecting on how I've never much minded looking younger than I am, even though it can be a PITA. I was a short, skinny kid (I'm an average adult) which probably contributed to looking younger...however, it also contributed to another mistake people used to make all the time. People thought I was a boy. I'm wondering if other people were mistaken for the opposite gender, too?
I loathed being mistaken for a boy; my gender identity was strong even as a toddler. When my mother would try to put me in overalls or something, I would fuss and demand the "girl's clothes," by which I meant dresses. I wanted people to be able to identify me as a girl. I remember at 10, shouting, "I'M A GIRL!" at a little old lady who had said to my brother and me, "Aren't you nice boys to go shopping with your mother?" (It was shortly thereafter that my mother let me grow my hair out for the first time.)
This never happens anymore, at least not when people can see me. However, it happens pretty often online. It used to happen in MMORPGs all the time, and I just figured that was because more guys play those than girls (although that's not as much the case as it used to be). However, it happens on forums a lot, too. I switched from my old online handle to Rose_in_Winter because I figure it's more feminine. I still don't like being mistaken for a guy...and I still wear skirts and dresses a lot of the time!
Disclaimer: this does not apply to transgender people, who often were mistaken for the wrong gender most of their lives, due to being born in the body of the opposite gender of the one they are inside.