Once worked for a place in the UK that will remain nameless. The building/campus had an exceptionally large UPS in the cellar (it was the size of a bus). The UPS powered every computer in the entire building, including client workstations, through a bunch of red 13A sockets with "clean power" printed on them.
We had a cold and dark winter one year that resulted in many issues with building power. It was eventually discovered that various office workers were plugging their 3kW fan heaters into the clean supply, and the poor UPS was having to resort to rolling blackouts to try and cope with the load. No amount of training, polite or otherwise, was sufficient to break this habit: we ended up having to swap all the sockets over to ones with a specially shaped earth pin to prevent a regular plug from fitting in.