Virtually unheard of (with black bears) unless you got between a sow and her cub(s), and then it's usually a false charge.
But it would take a hell of a steady nerve to wait & see if she was bluffing. 
Very true but I don't think I could sit and wait to see if its a bluff. Now I ain't saying i would hit the bear if charged. closest thing we have to bears in my area is wild pigs and I have some pics of 450lb ish pigs with 5 inch tusks they don't bluff if they feel threaten
