When setting up a holding tank for my parrot fish, I had just drainded all te water out of the aquarium.I said to my self what is that funny smell? smells like burnt toast?
So i put my hand in the tank (I was wearing washing up gloves and there was still some water in the aquarium)
Then FLASH BANG!!!!
the aquarium heat exploded in my face with bits of glass flew everywere! It was a real shock.And then I realized! I hade left the aquarium heat on when i Drainded all the water out.
So Always turn the heater of when doing water changes!

Untill then ALL things must be closed. This alongside other aquarium miths would make a owesame myth busters episode: can a aquarium heater kill you?