A couple had just moved in a house and called me to replace the ceiling fan in the living room. I pulled the flush mount cover down to start unhooking the wire nuts and noticed RG58 (coax cable). Someone had used the center conductor as the hot wire! I ended up running 12/2 Romex from the switch. There was no way in hell I could have hooked it back up the way it was. This is just one example I've come across.