My foreign body story
A long time ago in a computer room far far away we had a DECwriter II attached to a PDP-11. For those under 50 (55?, 60?), a DECwriter was a keyboard terminal, but with paper, not a screen. You could type stuff in using it, and it would print stuff out - up to 132 characters across the page.
Here's some info - http://www.computinghistory.org.uk/det/3367/Digital-DECWriter-II/
So all of a sudden, this DECwriter suddenly starts printing only 80 characters per line. This was very significant because, as no one under 60 (65? 70?) will know, 80 characters is the width of an IBM punch card. The IBM card became the industry standard, and, when computer terminals were introduced - both printer terminals and screen-based terminals - it was very common for them to be 80 characters wide.
Everything was working perfectly - except that it just wouldn't print more than 80 characters on a line. A longer line would be split into two. Annoying, and very puzzling.
So we spent a few days trying everything we could to find out what the problem was. Re-installing the OS (RSX-11M). Re-starting the machine umpteen times. Disconnecting and re-attaching the DECwriter. And so on. But the damn thing wouldn't print past the 80th column.
Then one day it occurred to me - a software guy - that maybe I should actually look at the device itself. I immediately discovered the problem - a pencil! It had fallen into the DECwriter and was firmly jammed in the path of the print head. By adjusting the position of the right-hand side paper tractor, you could use different widths of paper in the DECwriter. So our DECwriter was quite happily printing away, hitting the pencil, and doing CR/LF.
It was sheer bloody coincidence - down to the posiion of the pencil - that it was printing only 80 characters!