Luxating patella
A kneecap that slides out of its groove, making a dog skip a step or hold a back leg up for a stride before carrying on as if nothing happened.
The kneecap runs in a groove at the end of the thigh bone. If that groove is shallow or the leg is slightly bowed, the kneecap pops sideways out of it. The tell is unmistakable once you have seen it: a small dog trotting along, hitching one back leg up for two or three steps, then dropping it and continuing.
Vets grade it from one to four by how easily the kneecap displaces and whether it returns on its own. Low grades often need nothing but sensible weight and good muscle. High grades wear the joint down and are usually surgical, because a kneecap that lives outside its groove takes the cartilage with it.