"Popped" my kneecap this morning. The kneecap shifted out of it's socket and back in quickly, grinding some of the cartlidge. It's happened to me far too many times in the last 3 or 4 years. Hurts like hell when it happens but it's given me a day or so off work, so it's not all bad. :>
For the lucky people (like me) the kneecap slides back in, for the unlucky people it stays out (Trapt did it a short while ago, he told me). If it gets really frequent and painful, there's surgery available to shorten tendons or somesuch.. bugger that.
Needed a knee brace and crutches for 3 weeks or so, and am now doing a shitload of intensive physio so it doesn't happen again. It probably will though, according to every doctor, surgeon and physio I've seen.
DEFEAT.