I have a Shepard that I walk daily.  Only one dog in the neighborhood has ever bothered it - a young pit bull.  The pit bull was off leash in the owners front yard and ran two blocks to "greet" my dog.  The pit bull's owner started panicked  screeching apparently in an effort to get their dog to come back but that merely alarmed their dog and a 5 second dog fight ensued that left my dog with a thumb sized hole in her hide.
I'm not anti-pit bull, but wow - I have no doubt that some people shouldn't have pets.