When my Mum was a lab tech at a school, they had seperate cleaning areas for "dry" and "wet" cleaning.
One day someone managed to leave some sodium on a piece of equipment and left it in the "wet" area.
My Mum didn't spot this, and rinsed it off in the sink, the piece of sodium went down the drain and then blew a hole in the water trap much to her surprise (and everyone else in earshot)