The last time my kneecap dislocated (they like to do that):
When i was in sixth form three years ago i had finished lesson, everyone had virtually gone except teacher. Chairs don't fit under the tables, so i was squeezing between them, right kneecap caught table leg and out it came. Teacher and two other girls who heard what was happening managed to get me into a chair, and ambulance was called. Knee normally goes back in of its own free will, but it wasn't going to this time. Kept getting spasms up leg and knee travelled further and further around from where it should be. Was out for twenty minutes before ambulance came, gave me some morphine and popped it back in again. Was taken to hospital as a precaution, Mam was phoned at work telling her i was in hospital but not why I was in, so she was panicked until she found out it was just my knee.
It may not be an injury that leaves a scar or takes ages to get better, but it bloody hurts!!!!