In a mechanics lecture this pm the professor gestured wildy and smacked the head of the overhead projector down.
He didn't realise what he'd done. Then for the next ten minutes before the lecture ended we could hardly see what he was writing.
Only one person at the back spoke up and said "Can you move it up please?" The professor apologised and moved the page up instead of the head up and the student didn't go on to explain what he had actually meant.
You could see people agitated that they couldn't read what was being written. How come no one spoke up?