I used to like jerking off in a public restroom stall, leaving the door "accidentally" unlocked and waiting for some unsuspecting person to catch me. Then I act surprised, or better yet, if I wear a baseball cap and look down at my crotch they can't see my eyes, so they don't know if I even saw them. Most times they just quietly close the door.
My favorite stalls are the handicapped ones, because the door opens out on those so the person is guaranteed to see you, whereas with a door that opens inward they might open it just enough to see your knee or shoes and then close it without catching you "in the act". It's especially good when the stall is positioned so you can see who is coming into the room first so you can decide if you want them to catch you or not.
I don't do this much anymore, but it was a turn on for a while because you don' t know if the person walking toward the stall will open it or not. Many times I would come just from the anticipation of it all, though sometimes I actually did get walked in on. Only once was I able to time it so I was actually shooting a load into a wad of toilet paper when some guy opened the door! I looked up and pretended to be surprised and then covered my cock with the tissue in embarrassment. That was the ultimate turn on for me. I think after that I stopped doing it.
There's something about the public humiliation that I like, and the exhibitionism...
Another thing I used to do is, when wearing loose cargo shorts in the summer, wear them a size big in the waist (with no underwear) so they can slip down. Then I make sure there is a lot of weight in the side pockets, like lots of loose change or whatever, and let them fall down in a public parking lot while I'm walking. Sometimes you have to open the top button for this to work (it's even better if you're carrying something with both hands because then it takes you longer to grab your pants to pull them back up and the onlooker gets more of a show). Since I'm careful not to do it in front of children or ladies, I'll do it in something like a Home Depot parking lot early in the morning or somewhere where there are mostly guys. Once, I got out of the car in the rain and started running for the main entrance and my pants fell down all the way to my ankles, right in front of contractor who was taking stuff to his car. He got a good look. I was half hard and hanging down and although my cock is only 7", I'm only 5 feet four inches tall and 130 lbs, so it looks huge on me! I still think of the look on his face now and then when I'm jerking off...
OK, I know I'm a sick puppy!