When I was about 17, I was over at a friend's house. We were going through his porn magazines, looking at the pictures and reading the dirty stories.
I felt the urge to use the bathroom and headed in there. I sat down on the toilet. It was taking me a while and I was not only bored staring at the walls (I hadn't taken anything in to read), but I was still pretty horny from the stimulation of the magazines.
I started to stroke myself, with the aid of the generous pre-cum that had built up in my pubic hair earlier. I brought myself off and was finishing using the toilet, when my friend knocked on the door and sounded annoyed that I had been in there so long.
Apparently I had been in there a lot longer than I had thought. I got cleaned up and headed back to his room. We never spoke of it, but I think he realized what I was doing in there.
Not my finest hour.