This story begins years ago, as mentioned there. I’d retrieved the offending outlet expander / extension cord from my mother’s apartment and tossed it in my big box of Extension Cords.
I recently plugged it in and was rewarded with a flash-bang inside the box. Taking it apart reveals two more blackened outlet compartments (in the lower right), but no more missing contact blades.
It turns out that the black (hot) wire got caught between a stiffening rib on the back plate and the edge of the box supporting the brass plate connecting the white (neutral) wire to the contacts. Here’s reconstructed view after I cut off the extension cord.
Flipping the wire over shows the spot where the copper conductor eventually poked through the insulation.
It touched the sharp corner of the brass strip just to the left of the divider in this view. The notch in the divider channeled the jet of burning debris across the far wall of the right-hand compartment. The left-hand compartment is completely smudged.
Looks like I get credit for this one… but even seeing how I did it, I’m not sure there’s any way to know none of the wires got crushed while reassembling the box.
It’s safely in the trash and the cord is in my big box of Random Power Cords.
Memo to Self: Make sure the box fits together smoothly?