Even shihans dont have eyes in the backs of their heads. Get the drop on someone, they're done. Armed, yudansha, military, cage fighter, power lifter, gangster, psychopath, all fall down with a 2x4 to the back of the bean.
I got hit with a jo on the back of the bean once. Sucker was moving, too. Fortunately only a glancing blow, or I'm sure I would have gone down, instead of giving my dojo-mate an annoyed look. We were both incredibly lucky.