My last call of the day... a simple wireless network setup. I've done hundreds and hundreds of them.
I solved the immediate connectivity issue, and went to establish a remote connection, as I always do.
The customer ran our remote connection software. His screen went black. The mouse froze.
Nothing could unfreeze his computer. So he restarted it.
After restarting, no video, no display whatsoever. Not just after Windows loaded, but even before: no startup info, no BIOS info, no hardware check info. The screen stays black.
I had him check the monitor cable, turn the computer off, turn it back on. Nothing.
I had him leave the computer unplugged for several minutes, then retry. Nothing.
The thing is, software can't do that. The only thing that could completely hose video like this is a hardware problem: a bad video card, a bad motherboard... maybe bad RAM.
But the customer refused to believe that such a coincidence was even possible. And was understandably convinced that I broke his computer.
And, that's all. There's no real recommendation I can give. As it is I probably said too much: by suggesting that his video card might be to blame I stepped well outside my allowable scope of work. So not only is the customer angry... but there's a good chance I'll be scolded by my supervisor too.