The default GNOME desktop background (Adwaita theme) got corrupted somehow – initially with abstract blocky patterns; later, snippets of other things that had been shown on my screen and (apparently) other random graphics-related memory (some font tables?) started to be included, too.
The glitch is, when it happens, remarkably stable. The background does not flicker, and switching to a different background and then back to the default preserves all the artifacts as well. Sometimes GNOME switches from the “day” to the “night” variant of the background, creating a new variant of the glitch in the process (the “night” variant seems to leave more of the artifacts visible) – compare images 1 & 2 or 4 & 5.
Since this bug reappears a few times a month, this is a series which will continue to receive updates – until the GNOME bug that presumably causes it is fixed, I suppose.