I've had it happen on the exercise bike because i was wearing a pair of boxer briefs and somehow my dick squeezed to the side so that it wasn't in the pouch but stuck between the fabric and my leg. The peddling motion would then cause a perfect amount of friction for me to get hard haha. It sucked! so i was trying to peddle really slowly, but that didn't help, so eventually i was like fuck it and just got off of it w/a semi. Lately I've been having issues w/the treadmill and boxers haha. I don't wear button fly boxers so i always end up w/my dick outside of the fly when i'm running which is really irritating because it's hard to get back in my boxers without anyone noticing. *and sometime i have to get it back in b/c i wear the mesh-like shorts sometimes* It's a lose lose situation for me, the boxerbriefs suck and the boxers suck...I need to get a jock strap haha.