Lived with a absolute knob jockey of a warden at university, he bought a fresh little pint of milk every day as he was properly weird about off milk, we clocked this trend early on so also started buying little fresh pints of milk daily.
Ours would sit in the sun for weeks and then we would take the labels off of his new milk put on our old milk and replace in the fridge.
I don't think he ever figured out what was going on and it send him loops :)
The one I feel most guilty about however is starting a little fire in the corner of a corn field in Cheshire, being young I had no idea that it would spread, and spread it did, across the whole field, to neighbouring fields and beyond!!!!