This reminds me of the time I happened to be at a warehouse where an industrial motor control panel was being decommissioned. In the center of the panel is a large breaker, which was dutifully opened (ie powered off) before work commenced. But bizarrely, someone in the past managed to tap power from the supply-side of the breaker for some sort of monitoring sensor inside the panel. So when that circuit was cut through, there was a loud bang and the overhead lights went out.
No one was injured, although everyone was jumpy from the inadvertent light-and-sound spectacle. And a set of cutters gained a 12 AWG-sized (approx 4 mm^2) hole.
I may have misremembered some details, but my takeaway as a non-electrician was to 1) never assume a breaker handle at face value, and 2) don't assume the prior person made sane choices.