It almost happened to me once during a threesome. I was riding a guy with a smaller dick (maybe about 5") but it was super hard. I was going to town humping his dick and I felt a kind of pressure start to build up down there, but he made me stop because he was going to cum. Just a few minutes longer and I would have SO gotten off. That was the first time I had my prostate stimulated (I don't get fucked a lot) and it felt amazing!!!