Ah yes the sadistic sensei! Many common experiences have already been pointed out in this thread (like being walked around the dojo in some sort of lock - or worse only your (previously injured -) fingers in some sort of lock
I do have a couple more: How about a sensei who also has a jujutsu background and likes to demonstrate a few grappling locks and pins.
"Oh he won't be able to move in that possition. My butt is sitting on his head!"
Yes. Thank you for pointing out the one element of this pin I was trying so hard to repress!
Using a real sword: "Try and pull my sword!" Stupid as I am I reach for the hilt, expecting him to apply nikajo (nikyo) using the hilt of the sword (which we had practiced before). But NO!! Do you have any idea what it feels like to have a cast iron hilt rammed into your stomach hard enough to lift your feet off the mat?
Yet, all of this is all right. I am studying martial arts I need to toughen up a bit out on the streat your attacker won't ease up on you, better get used to it. But then there is the psycho-terror.
At the time I was an 9th kyu (body control? What's that?). Using wooden swords and practicing front-head strikes. My "blade" always wobbled after the strike. Sensei walks up to me stands in ffront of me: "RAISE FOR HEAD STRIKE" I raise. "Don't hit me!"... Let's just say I have no recollection of the actual strike. Somehow my blade had stopped less than 1/2 an inch from his forehead, my knees were weak for the rest of this session and I had bad dreams for at least a week after.