As a teenage masturbater (17 years old) I would try to come up with new ways to enjoy myself. I used to sneak out of the house by climbing out of my window and walk to the 7-11 store and buy a Penthouse magazine.
One night after I had myself as hard as I could be I thought it might be fun to try jacking off outside. I had always just done it on my bed, squirting onto my chest. It was after midnight and I thought it would be safe to do it in the backyard without anybody seeing me. I climbed out of my window and took off my underwear and started stroking my boner. It felt weird doing it standing up. I was super horny and got brave enough to go out past the back fence into the alley. I was in a place where I could be easily seen if it were light, and there was some light coming from the neighbors garage lights. But I was sure I would be safe since it was so late. The idea that I might be seen or get caught both scared me and made my boner even harder. I was just starting to come when a car turned into the end of the alley. I was too close to coming to do anything but move behind the corner of the fence and keep stroking. I was sure whoever was in the car could have seen me in their headlights as they turned into the alley before I ducked behind the fence. I didn't know who it was in the car. There was only one neighbor who needed to drive in the alley to get to their garage. I was good friends with Larry who lived there. We used to sneak into his brothers room and read the Penthouse magazines he had hidden. If it was one of his parents I would be very embarrased, and probably in trouble! By the time the car came to the corner of the fence where the driver might see me again I could see through the fence slats that the driver was Larry's younger sister, Mary Lou, who was one year younger than us. She was alone in the car. The thought of being seen jacking off by Larry's hot sister just made it more intense and I didn't even try to hide better. She took a quick glance over to where I was only somewhat hidden. There was enough light coming from their garage that I was sure she had a good enough look to recognize me. By then I was in the middle of the best orgasm I had ever had, squirting sperm all over the place, and was almost hoping she knew it was me. She pulled into their garage just as I was finishing. I moved a bit more behind the fence and crouched down to hide myself a bit better. She came around the back of the car and looked over in my direction but couldn't see me where I was hidden. She went into her house. I climbed back through my window and got in bed.
I was scared that she might tell her parents and that I might get into some kind of trouble. Fortunately that didn't happen. But it was obvious that she had seen and recognized me when Larry mentioned to me the next day that she had asked him if I had a new job or something that would bring me home late as she thought she had seen me in the alley the night before. Larry knew I didn't have a job that would require me to come home late, and so he wondered what his sister was talking about. I told him the truth a couple of weeks later. He cracked up laughing about it but kept it a secret.
I've been able to jack off thinking of that night, and other nasty things about Mary Lou and the other neighborhood teenage girls, many many times over the years.