This may be something that is heard over and over again, but I'd like to share this story as a practical example of a "could have been."
A newly-purchased house had some water staining on the ceiling of the room below a bathroom. The damage appeared "old" (not wet, not leaking).
The leak began to appear again, so the owner and a family member were going to open up the ceiling to see what the cause could have been--perhaps an easy fix.
Fortunately, they tested for AC behind the plaster and lath (in this case by using an inductive probe which as a side-effect, emits a nice hummmmm if there is AC present) and sure enough the hummm was there. (NOTE! Some newer units are designed specifically to filter out the AC hum! There are testers specifically available for detecting AC.)
As the owner was somewhat familiar with the wiring layout in the house from some other work performed, power was shut to the circuits that used Knob and Tube wiring (all of it was connected to 2 circuit breakers for the whole house...) Testing again revealed no AC hum at all once power to the K&T was cut.
The surprise came when the ceiling was opened up. Aside from the rot at the top of the toilet elbow bend, (that was left to a plumber to fix) there was a completely bare conductor which apparently, having had water dripping on it, lost its cloth covering and rubber insulation over the years. (At least with some conductor insulation, someone stands a chance of protection if they are poking around in a wall or ceiling...this conductor was naked!)
All of the Knob and Tube circuits were kept out of service and fortunately there were no injuries from any of this.
Unfortunately my photos of this are lost due to a prior hard-disk failure.
Lessons learned: ALWAYS check for "surprises" before drilling/cutting/reaching in/etc.
ALWAYS use your protective equipment and test equipment.
(And always back up your important data!)