One time, back when i was doing engineering, our teacher was drawing an orange on the board for some reason.. He drew a circle, looked at it for a second, then goes "No wait, i'll draw it from the other side", rubbed out the circle, drew the exact same circle, then just went along with the lesson as though he didnt do anything stupid..
Needless to say, we all cracked up laughing and did nothing else that lesson
It was the same teacher that, when driving a minibus down to newcastle for the engineering excursion, hit a van and drove off.. When the owners drove up along side the bus they yelled out the window "You hit my van!". So, he wound up the window and drove off along a side street and lost the van