My in-laws food mixer was plugged in to mains power (turned off) and then suddenly started working. I'm guessing there was a small voltage spike on the power line which caused the arcing, but i have no evidence. The arc connected a live mains wire on the pcb with a relay, causing the engine to work.
First, see this log update - A zombie food mixer
Then, see this log update after the fix - It lives to fight another day!
You can see the problem and fix in the pictures of this project.
Lastly, as i didn't make a video of the device tear down, here's one my father-in-law found in russian - Repair of food processor BOSCH MUM5 ... replacement gear reducer - (no, we do not speak russian).