I had a very strange monitor bug a few years ago. Dwarf fortress and a weird linux display mode drove my monitor to display a freeze frame of what was going on during the bug at about half opacity with a bit more of a glitchy feel.

It stayed after reboot, mode changes, OS changes, retriggers of the bug etc. etc. etc. to the point where it did not make sense at all how the pseudoimage stayed past power cycles and everything else I could imagine. I briefly, and quite seriously, questioned my sanity trying to figure out what was going on.

Eventually it just went away. Some firmware bug got an image stuck in memory or something, but man it was weird.