Recent fault I attended to in a small block of flats. When the outside light by the front door was switched on, all the remaining lighting in the home was switched off! It was dark at the time I attended which made it even more of a surprise. Someone previously had decided to repair a broken plastic back box on the outside light switch with end result that wires were put in the wrong connections on the switch! All the time the outside light was not used there was no problem with the in home lighting.