Not only high end systems...
I must confess to a similar "lapse of memory"! Hark back to the days of the BBC Micro, which adorned a desk at my home, topped by a colour monitor!
One day, upon switching on, I was presented with a very monochrome display! The colour had simply disappeared, and I was looking at a screen of various "Shades of Grey" - to coin a phrase!
I checked the video cables, all OK. Checked the output of the BBC Micro on a "portable" colour TV. All OK! Concluding that it was a monitor fault, I dug out the circuit diagram and discovered that all the colour processing was done by a single, rather large, chip. It took a few days to locate a supplier of said chip, but I finally had one in my hands, and took the monitor in to work at the week-end to avail myself of the very well-equipped workshop.
After a lot of cursing and swearing while extracting the main board from the monitor, I finally managed to replace the thousand legged chip (well, it seemed like a thousand!), re-assembled everything and switched it on. Everything was still a glorious monochrome!
At this point, one of my colleagues strolled over, having heard my cry of anguish. He leaned forward and turned up the knob marked "colour" on the front panel! Needless to say, normal service was immediately restored!
In my defence, I can only say that I had not touched any of the front panel controls between the thing working and not working. However, I did have two small and mischievous children. I do not know for sure that it was them, but the way they giggled when I finally announced that I had found the problem made me watch them much more closely thereafter!