I've had a couple of people who have told me my left knee goes out on the pedal stroke. I think they are only trying to be helpful, and I accept the information as friendly. (I ride by myself and have a social life that rivaled the unibomber, so I am not exactly predisposed to this kind of interaction.)

Unless it is meant as a put-down or something, just take it for what it is -- information and opinion. My bike is custom fit and my frame builder mentioned one of my legs was slightly longer -- I can't remember which, or even if I had this issue before I broke my ankle. It is just life.