I think it's sometimes good when senseis are "sadistic" - you will really REMEMBER your mistakes when they are pointed out not only by words but with a nice atemi.
I have been walked through the class with sankyo
or some jujitsu nikkyo thingie. Also been choked from the rear when my control was not right being punched when I had a hole in my defence and of course there's always the "please try to stand up" thing (while your hand is still in yonkyo, nikkyo etc) And when being a lame uke (not stiff and "real" enough to throw i have also encountered some atemi-based throws or controls instead of throws.
And this is good! Of course he does NEVER cause an injury this way. Just sometimes leaves you hurting a little
There is somewhere a thin line about that subject and when your sensei stands on the "good side" of that line... but really close to the line. Probably it'll help you learn the most.