There are plenty of work-related ones I remember. There was the water cooler which had a hazard sticker, with the legend "Caution. Dihydrogen monoxide. Fatal if inhaled". It was rumoured that two successive managers had ordered it to be moved somewhere safer.

I worked with a hardware engineer with an insane sense of humour. His favourite trick was to enter a room with an empty cup and pretend to trip and lob it in the direction of someone. They would invariably leap up, thinking they were about to be scalded. Working at a customer site one day, I received a garbled telephone call about a software failure at another site. I rang my office for details, and the joker picked it up. "Gary, I've just had a call about a software error at ... Hospital. What is it ?" "It's a big building full of sick people, but that's not important right now". Click.

An April Fool which my father told me about. Somebody at University rang the Police and said, "There are two students painting the front of the Town Hall". The Police came rushing, to find two art students with easels and canvas on the pavement on the other side of the road.