He came trotting in through the cat flap at about his dinner time, and went upstairs.
C went up to bring him down to be fed, and put him in front of his bowl, in which he seemed somewhat uninterested.
He ate a bit, and then came out into the hallway and sat down, with one rear leg pointing in the wrong direction.
We think he'd have had difficulty getting in through the cat flap if his leg had already been like that, so somehow, in the course of five minutes, he managed to either dislocate or break something, and he didn't even yelp.
My current hypothesis is that he managed to pop his left hip while coming in through the cat flap, but we're waiting on the X-ray.
(As far as breed is concerned, Abyssinians are energetic and athletic, and certainly not known for hip dysplasia)
ETA: no, it's a fracture of the femur at the growing point. The leg may have finished growing, but it apparently takes time to finish toughening up and meanwhile his musculature is already full strength. So he appears to have managed to snap it under his own strength.