Answer by Helene Hoegsbro Thygesen:
At my first job as a programmer for the Danish Road Department, the software lab made a mirror of the information service's production system so we could test the software in a realistic environment before we delivered.
Once, a program that always worked correctly in the test environment turned out to give garbage output about half the time in the production environment. After some testing in the production environment we noticed a pattern that it turned out to work when operated by specific production staff, so I went for a couple of days to the production site and watched the production staff while they operated it. It turned out that when one of the unlucky staff used it, it also failed when I used it instead of him. So apparently it was about the shift he worked in, not the operator himself.
It turned out that the software worked only after 10am. Since the engineer who made it and tested it in the development environment never came to the office before 10am, that wasn't discovered before it was shipped to production. He made a temporary file with the dos extension ".10x" when dumped between 10 and 11, ".11x" between 11 and 12 etc. When run between 9 and 10 it would be ".9 x" because the string padding routine would right-justify numbers, and the routine that read the file in later could not handle spaces in the extensions.