Fortunately, I don't think the fact that there was a sign about it means anyone actually did it, but you never know. I attributed it to scientist humor, but then, my father had to tell his officemate that it was impermissable to keep hazardous chemicals in the office freezer.
I have seen precipitated milk. I admit I've even owned some (usually when I've gone away and told people they're welcome to drink it while I'm gone, and then they don't). I can't quite picture leaving precipitated milk in a communal refrigerator, though.