Totally unintentional on my part, but I made a guy cum while prepping him for a PA piercing once. It was this kid who had been bugging us for months about getting work done, but he wasn't 18 yet. As soon as his birthday rolled around, he came in to get it taken care of. I was swabbing his dick a little, he popped a boner (which is pretty common), and boom my shirt had a nice stain on it. The whole thing took maybe 10 seconds.
The kid pulled his pants up and apologized profusely. I tried to calm him down, but he was so embarrassed that he insisted on paying for the piercing without having it done. Kind of made me wonder what I'd just been paid for, but he wanted to get the hell out of there. Sad thing was, he was a cute kid. If all he wanted was a hand job, he only had to ask.