Oh I agree. My dog did it once though, a young Boxer who I later found out was well known for really p*ssing off other dogs with his behaviour. Shadow had never met this dog before and it wasn't like him to go immediately for a high level warning so I wondered whether he somehow picked up on this dog being very confident but so completely clueless about social niceties.

Not saying that was the case in this instance, the other signs (dog approaching slowly, staring etc) would make me think either a dog with aggressive tendencies or a dog who was uncertain and wary of the reaction it would get.