Ripped a ligament in my right knee while skiing. I'm still feeling it today. As soon as I fell, I felt and heard it rip. Let's say I wasn't happy, as it pretty much ended my ski season (couple of weeks salary off, as I was a ski instructor on winter back then), plus I'm still having trouble squatting - I have to compensate by putting a lot more effort on my left leg. It wasn't ripped enough to go through surgery, but enough to leave some limitations.

Close second would be when I got that circumcision - no guy should have to see his penis blue and bleeding. :P