Users - don't you just love them!
While working for a government department, I was the only person who could support a legacy system used to receive overtime claims via modem link, which were then fed into a mainframe based payroll system. This ran on an old CCP/M system in the datacentre, while I was based in another office about 5 miles away.
One day I got a panicked phone call to say the system had gone haywire and they needed to get it working before that night's payroll run, or there would be huge ructions if the overtime was not paid on time.
I asked what was wrong with it and was told that most of the menu items had disappeared. Thinking that the programs had been corrupted in some way, I gathered together the backup disks, manuals etc and jumped in a taxi.
After being admitted through the airlocks, I examined the system, turned up the monitor brightness, then walked out again.
It turns out that they had had some IT cleaners in, who were to clean the monitors, mice and keyboards. While cleaning the old green screen monitor, they had accidentally turned down the brightness/contrast so that only the highlighted menu options, shown in reverse video, were visible!