Some years ago, I worked for a small tech company that supported various clients in North America. I was assigned to provide on-site client support at a midsize warehousing operation.
One day, I got a call from the foreman that a line printer located in the warehouse had inexplicably gone out of whack. It was skipping lines and printing lines on top of other lines, to the point that the document wasn't legible.
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I went to the warehouse to investigate, but couldn't find anything wrong. Later that day I got another call: same problem. Again, I couldn't find anything wrong. I was certain that it was related to hardware, because as a programmer I was intimate with the software driving the print process and saw no way that could be the issue.
After a couple of days of the printer going out of whack, the warehouse foreman and I had a plan to catch the print problem "in the act." I told the foreman to watch the printer and let me know if he ever saw anything unusual, and to call me at my desk when the printer started acting up. When he'd call, I'd run out of my office and 100 yards across the lot to the warehouse, sprint up a couple of flights of stairs, and dash over to where the printer was located.
However, every time I got to the printer, it would be printing normally again. This went on for months, happening once or twice a day.
One day I happened to be standing near the printer for another issue. I noticed a warehouse worker walk up to it when a print job was in progress, look at it, shake his head, repeatedly press the Line Feed button, then walk away. I went closer to the printer and watched. About 15 seconds later, it started going crazy again.
I asked the warehouse foreman to page the worker, "James," who had just left. When James came in I asked him what he was doing at the printer. He said, "Oh, I'm speeding it up. Pushing that button makes it print faster."