Dot Matrix printers depend on micro-switches to control behavior. A trouble report requesting a service call on a dot-matrix printer was one of the strangest ones I had ever read. The report said, "Missing platen".
The platen is the round rubber roller behind the paper, to both help the paper advance and give the pins something to push against. Getting to the site, sure enough -- no platen. The paper out sensor had filled with paper dust. It couldn't operate to shut off the printing.
The paper thickness sensor simply 'thought' it now had thinner paper, moving the print head closer. With no paper the pins started beating up the platen. As the platen wore down from the beating, the same paper thickness sensor moved the print head closer, so it could continue to eat the platen.
Based on when the office started the job, I figured it beat away at the print job, sans paper, from 7 PM to 9 AM. By then, the platen was a bunch of chewed up bits of rubber on the bottom of the printer and the floor.
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