I always test it before I touch it. My son owns a lot of properties, and one had a well pump that needed service. I found a breaker that shut it off, so I turned it off. It still had power though. I found another breaker that also powered it. It was 240V and they used two 120 volt breakers with 10/2 wire with the breakers not side by side/linked together so I had to find the second one.
In another of his houses, I turned off and then on every breaker one at a time and could not power down a circuit. It turned out that the circuit was fed from both ends, someone had added a few plugs on another breaker and ran a wire to an existing plug, so the circuit was fed from two breakers. I eliminated one breaker, pulled the wire out of the box and since it was live from the other breaker, I used it to put a light over the panel.