Rotate the monitor
In another life I was a trainee TV engineer back in the last days of valves and swap-out printed circuit boards. When engineers worked on a TV in the workshop there was always a mirror built into the workbench so that you see the screen while working on the TV's innards - of course, the view of the screen was always back to front but we got completely used to that (which probably explains why I can still write backwards). On many TV sets the scan coils that drove the CRT beam horizontally were connected by a couple of single wire connectors and swapping them around would show the picture back to front, so that it became the right way around in the mirror. Pranking this swap while the engineer happened to not be working on a set was not uncommon situation and there were several instances of a set getting as far as the customer's house before the prank was spotted (usually by the customer who was unused to seeing things the wrong way around).