First post here! We bought this old (1925) house late last year and have been fixing things up. Went to swap out a ceiling fixture in our finished attic bedroom, flipped on the breaker, and despite even the wall switch being "off," my wife screamed that sparks were flying out of the J-box!
I'm not electrician (obviously) but I swear I just replicated how the previous fixture was wired. The box has two old knob & tube wires (one with a black sleeve, one with a tan sleeve), and what looks like conduit with the standard black, white, and green wires. I connected the k&t black/conduit black/fixture black together, the k&t tan/conduit white/fixture white together, and so on.
After realizing what happened, I checked the damage. The conduit black/hot wire apparently snapped in half inside the box. I'm not sure if I damaged it somehow when moving the wires back into the box before putting up the fixture, or if it's possible it snapped from the energy release?