Was working as a telephone fitter in the early eighties, and went to a client's premises to fit a wall-mounted phone extension.
The client had decided where he wanted it, and helpfully marked the wall with a big cross in marker pen. It was in the middle of a bare external wall, with no electrical sockets or switches anywhere near it, so I was happy to fit it where indicated. (This was in the days before cable finders were a thing).
So I marked out the bracket, drilled the first hole, no problem. Started drilling the second hole, big flash and bang... Oops!
Turns out some electrician, in the dim and distant past, had run an electrical cable diagonally across the wall from top to bottom, and then plastered it in with no steel capping or anything over it.