I can confirm that you get a loud pop followed by funny burning smell and a few blobs of red plastic on the bench when you connect a single red LED across about 20 (whatever I found in battery cupboard in the physics lab) PP9 batteries wired in series.
Connecting a car headlamp bulb to the mains is not a good idea. You do get nice bright light which you will still see whenever you blink for the rest of the day but bulb life is rather short....
It wasn't me but someone thought it would be fun to leave the chuck key in the lathe in the metalwork room and start it up. The caretaker was most unhappy about having to replace a window frame...
Turning of the fuel cut off valve on the bus belonging to a visiting hockey team was good fun when the bus airlocked going out the school gates, blocking the onl;y entrance and the headmaster couldn't get his car out to go home
