Put a new mirror up in the hall. Now I can check that I look my absolute handsomeist best before leaving the house. It's a very optimistic mirror.
We carefully decided where it needed to be hung and, just before I fired up the drill, I ran my trusty stud-finder over the wall. I bought this a while back. It's a metal detector for wall use. It tells you if you are about to drill into a pipe (messy and expensive) or a live wire (messy, expensive and fatal).
Anyhoo, it bleeped in a fairly conclusive manner right over the spot I'd marked. Turns out that the wires from my mains sockets go up to the ceiling rather than down to the floor. So the mirror is four inches to the left of the optimal positon. And I'm not dead.