I've just remembered an incident from when we took over our house nearly sixteen years ago, there was a pair of wires poking out of the plastic panel by the front door which I had been told was for a door bell, there was a switch next to the front gate so lacking my multimeter I assumed the wires were just switch wires so I bought a battery door bell and screwed it to the wall and connected it up, got my wife to press the switch by the gate, there was a massive ding followed by a massive bang followed by a lot of smoke, I still can't believe that the builders ran mains voltage to the gate switch.