i was born with lateral sub-luxating kneecaps. they float higher off the joint than normal knees, and the way my ligaments attach around/over it are off to the outside of the knee, so given the right conditions the patella slides off to the side instead of just floating and moving back and forth like its supposed to. Picture the kneecap sliding to the side of your knee joint - pushing your bones into a slight V position instead if straight up and down. I could have surgery but it'd be major surgery with at least 6 months recovery for each knee.

Both of my knees have it, but my right knee was always the one that goes out - I'd say over the course of my 35 years I've dislocated it roughly 50-60 times. Sometimes its running, sometimes walking, sometimes standing and turning wrong. Sometimes when it's a sudden movement like slipping on ice. You just never know. But because it goes out so often it doesn't barely even swell anymore and after 10 minutes I'm back to walking like nothing happened. Those nerves and cartilidge is so messed up my body has adjusted over the years.

But my left knee - this was only the 3rd time. So, that sucker isn't used to it, and it's bad. Was probably softball size swollen. hairy leg pic:
This is one reason I cycle - becasue my knees prevent me from running, or engaging in most sports where you have to move around quickly and pivot on your knees. I golf, I bowl, and I cycle. I never fully extend my leg when pedaling so there is less risk of a dislocation. *knocks on wood*
(also why i don't ride fixed)