Patella Alta
In patella alta the kneecap rides too high up on the femur (thigh bone). This happens in quite a few people and may never do any harm. However, at the higher end of the thigh bone the groove in which the patellar slides is much shallower than it is in the normal position, so people with patella alta are more likely to dislocate their kneecaps.
There is a nifty animation here: http://www.carletonsportsmed.com/Pat_alta/patella_alta.htm