Mixed breed dogs are difficult to ID. Is it a boxer mix or an American Bulldog? Is that a Rottweiler mix or just a really blocky Labrador? Phoo. Though I have to say that unless the people in this study here were given no scaling clues, I don't see how anybody could mistake a Shih Tzu for an Old English Sheepdog.

http://www.denverpost.com/portal/news/ci_14005785?_loopback=1