Dimness
A long time ago.... I used to get calls that a monitor was blank. After having them check all the usual suspects (is it plugged, power light on, etc) I would end up driving to their site.
It turned out brightness was turned down as far as it would go. Easy to fix and of course no one admitted to touching tne brightness button.
As time went on I was still getting those calls and it was always the brightness (or dimness haha). It turns out they were accidentaly pressing that button whenever they moved the monitors. The button was located under the front edge of the monitor which was the perfect spot to grab the screen when it needed moving.
At work we would refer to it as the dim bulb problem.