I had a bit of a weird and new problem the other day.
A customer had unpacked and set up their new monitor to their new Windows 10 PC.
When the machine booted up, sometimes we'd get a desktop but no icons or system tray items.
The machine seemed to be working but it just never seemed to start up properly sometimes.
I'd noticed that the customer had used the blue VGA cable to connect the PC to the screen, but when it powered up I noticed (somehow) that the HDMI input was involved.
So I checked the back of the machine and the monitor to find that they'd also connected the HDMI output from the PC to the corresponding input on the monitor.
So, 2 connections from one PC to one monitor.
I twigged that the monitor was in fact trying to "display" both outputs from the PC but, obviously, we could only see one at a time.
I'd not seen this before that I recall.
Normally monitors only display (or accept input from) the chosen input source.
This monitor (not the one pictured by the way, that's just for illustration purposes) had chosen to accept all input and keep them active but just SHOW the chosen input.
So just to be sure, in the future, it is probably best if you only connect one video source from a PC to a monitor at any one time. :-)