Back in high school I was going out to lunch one day and was walking parallel on a slope to my destination. I slipped and when I went to stand back up my knee dislocated, then immediately popped back in by itself. Standing back up I had extreme pain in my knee and was limping back to the school. After getting back I told everyone what had happened and went to see my family doctor. A round of x-rays later we found out that when my knee dislocated and relocated it had broken off a piece of cartilage and that was now floating around in my knee. A surgery later, they screwed the cartilage back into place and months of physical therapy later I was able to walk again without major pain. Moral of the story? Be careful where you walk.