Basically, I had my dick tied up super tight, and then at some point I flexed the PC muscle (the one that twitches when you have an erection and when you ejaculate) while my erection was at max. There was a quiet but audible "pop" sound, and ever since then I've never been able to get fully hard for more than a couple of seconds during orgasm - it grows up to full size, but stays soft enough that it'll bend if any pressure is applied to it, which makes penetration nearly impossible.
I've had two MRIs on it and my urologist has said that it might be possible to operate on, but it's a pretty delicate area, and if anything were to go wrong there's a very good chance it'd end up causing even worse damage.
It's never really hurt, but for a few months there was a pretty big blood clot near the middle of the shaft which wasn't painful, but just felt very uncomfortable.