First time it happened to me today.
I'm not out yet or anything, and I don't present as female (yet), but I am growing my hair out and I wear girl jeans all the time. So today I went to my usual therapy appointment at the local LGBT clinic, and afterward I was going down the stairs to the lobby to leave, and some people were just turning to go up the stairs. I saw them and one of them was a guy. He saw me as he turned, and then he looked back at me again with a bit of the classic double take move. I have a feeling this is going to be happening more often in the future

.
After therapy I went to pick up my new glasses, and for the first time, contact lenses. I have been wearing glasses consistently for about 9 or 10 years now, so today was the first time in a long time that I got to see my face without being zoomed in 6 inches away from a mirror. I think I need to get a slightly stronger prescription for them though.
When I got home, the last of the items that I had recently ordered was waiting: my first pair of girl boots. ~4.5 inch heels, up to just under the knees, black faux suede. I put on my favorite skirt and a v-neck t-shirt, put on the boots, and then went to look in the mirror by the stairs in my house (no one else was home).
There was just one thing that needed fixing: hair. Mine is long enough in the back that I can twist it up into a clip, but the front side parts don't go back that far and usually just end up looking kind of messy. Then I had a little bit of inspiration and fixed up the sides with these hair pin type things I have. Instead of sticking them in horizontally above the ear from the front of my face toward the back, I stuck them in upward and diagonally *behind* my ears so that all the hair that normally curl-poofs out sideways was tucked neatly against the side of my head. Then I looked in the mirror again.
I did my own double take. For the first time, I saw a girl. Head to toe. I looked really nice. A little iffy around the chin and upper lip because I haven't yet figured out how to cover that properly with makeup, but the actual shadows were in my favor and it was easy to overlook that. Facial features could be better (mostly nose and jaw), but with money and time, that can be fixed.
Up until now I've been hesitant about saying 'I am a girl', partly because I have some issues with the normal definitions of gender (and sexuality as well), thought not because I doubt whether I have 'GID'. But I feel like after today I can say 'I could be a girl' and mean it. Maybe even 'I can be a girl'. I guess I'm being her right now, and really
really I don't want hide that away the next time I leave my room.