In the early days of PCs on office desks, I was called to troubleshoot a "dead" PC by a senior military officer at the place I worked. On the phone, I went through a lot of questions and the guy kept saying that the screen was blank but the machine was making a whirring noise; it took me ten minutes of this to realise that he'd switched the monitor off when he went to lunch and forgotten to turn it back on! Well, you'd tend to assume that he'd have noticed that, wouldn't you?
Mind you, this same guy asked me to get him a laptop. I asked him why, knowing of his limitations with computers, and he said that he was the only one at a meeting who didn't have one and he felt left out!
Ah, those were the days :~)