This just happened to me today.
I was on my way home from Toronto. We stopped at a rest station to take a piss. The washroom was full of people. I found a stall and went in to piss. The only problem was that the door didn't latch. As I was in there, I got really horny and decided to stroke my cock a little bit. As my cock grew and grew, I saw the door swing open and a guy walked in and pulled the door closed behind him.
He just stood there and stared at me stroking my hard cock. Eventually he asked if I was close. I told him that he was welcome to stay and watch. He did just that and as I was busy stroking away he asked if he could hold my cock. I said sure, but only if you stroke it like I was doing. He did and soon after that I came. I shot all over the place, and over him.
Then he decided to take out his large semi-hard cock and got it stiff quickly and asked me to do what I was doing to mine, on his. So I jacked him off and he spurted his cum into the toilet. I then left and continued home. It was the best experience I ever had, maybe I'll leave the door unlocked on purpose in future.