"OK class, today's lesson is going to be a dictation, get your books out and start taking this down"
I can't remember the exact content but effectively he started off with some familiar organic chemistry and then veered off to stuff that was pretty obviously wrong.
I'm proud to say that I'm the one that said "Eh ... that doesn't seem like a valid equation ... ?"
"Good! Never just copy this stuff down without engaging your brain!"
(Or at least that's the way it goes in my memory, 17 was a long old time ago!)