I had one once, I was using the chasing machine in a lounge for a TV mount. The wall was external and unbeknown to me the Gas meter box was on the other side.
So there I am with the chaser, going across the wall. When I could suddenly smell gas!!
I panicked, turned the chaser off as I thought it would make the gas explode; then set about running around the room!
I don't know why, I just didn't know what to do.
Then it dawned on me that it wasn't going to explode, and I should open windows etc.
And of course turn the valve off at the meter.
The Blinking plumber had come straight out of the back of the meter box, down the INSIDE of the wall then it had been plastered!
This wasn't some acient building before the days of regulations, it was a new extension (about 5 years ago).
I don't think I will ever forget this. Although I can laugh about it now