Being a janitor, I know better than to ever unplug any computer - desktop or otherwise. Granted, 99% of the places I've cleaned don't allow us anywhere near the servers, but I'd still never unplug anything. Even using the same outlet as a client's gear is a no-no.
That having been said, I once took over a site from a lovely older gentleman. My employer informed me that he was being removed because he'd received a complaint about the dusting of the desks. The problem wasn't the complaint itself, but the fact that he disconnected and moved everyone's PCs in order to do a very thorough dusting. Didn't shut them down first, just yanked the power cords and everything else that plugged into the back of the tower.
Poor old guy genuinely didn't understand why he was being taken off that contract.
Beer, because I desperately needed one after finding all that out.