Peek-a-boo!
School
This happened to me in the 7th grade.It was Gym day and we wore shorts to school. I had to make a speech in class. As I was talking, I noticed Kim in the front row. I could see up her skirt and see her sheer panties. This got me excited. I was flustered and couldn't talk strait, and to top it off, I got an erection and it poked out the top of my shorts. The whole class saw.
To this day, I don't like speaking in public.