One thing I've always been slightly puzzled about. My back room has two sets of lights. There is a double switch by the door, one of the switches only works the set of lights near that switch. The other switch works the other lights which have their own switch on the other side of the room, obviously a two-way switch. If both switches on the double switch are switched off there's no problem. But if one of the the two-way switches on the double switch is switched on and it's switched off the other side of the room on the two-way switch, there is a very faint glow from the lights on the other side of the room.
I know that sounds absolutely confusing, but if anyone can understand it, I wonder whether it's dangerous if I forget and leave the two way switch the wrong way round which leaves the lights faintly on.

Lawrie