first time presenting female was at london comic-con 2009, I felt great doing it (had waxed my whole body, and since I was shorter and skinner, I passed well as a girl. I was with some friends and there was a guy there taking pictures... only of girls, with camera at boob height (came off as a bit creepy). Anyway I spoke to him for a bit and he asked me for my number. I pointed out just for his sake that I was bio male (and ironically I was in a state of denial over my gender issues at the time).
He went very pale, turned and briskly walked away. I was laughing about that for weeks!
A more recent one. I went to the shops looking for my first pair of short shorts, found some really cute ones in the girls department and went to the changing rooms. I went with a female friend and the woman who was at the changing rooms (they arent signposted and it was my first time at the shops so I didnt know it was the girls) took the shorts straight out of my hand when I got to the front of the qeue and told me to wait while my friend tried them on (despite them being 6 sizes to big for her, shes tiny)
"wait, those are for me, actually."
She looked stunned, and the old dears in the qeue behind me just went very quiet. Thankfully one of the other members of staff came up to me with a smile and said: that's fine, but you'd need to go to the other dressing room, this one is for girls only. I apologized, took the shorts and went to the other one as the people in the qeue just watched me go. I tried them on, perfect fit, bought them. Best part was thinking how much of a natter the people behind me were probably going to have

two very different times, and yet both funny for me.