...has your sensei ever done something like that?
I've never seen him do it quite like that. One fellow in our dojo found out he was testing for shodan about 5 minutes before he did it. I found out on a Friday evening at 7:00 that I would be testing for nidan the next day at 2:00 p.m. No time to prepare, just enough to worry a little. Sensei likes to do stuff like that