Getting and relying on medical advice from breeders and groomers (with no medical background). I once saw a rat terrier with a fractured humerus, which typically requires surgical correction. As I stepped out of the room to check availability with a surgeon, the client called the dog’s breeder who said not to follow my advice and to “just put the dog in a sling” and that she’s “done on her own dog's plenty.”
Also, not exercising dogs enough. Many behavioral problems can be solved with ample exercise daily."