If anybody wonders (like I did) why their Garmin basemap cycle 20M1 download became non-installable or why, if already installed, the system considers 19M1 an upgrade over 20M1:
https://www.garmin.com/en-US/aviationalerts/svt-colors-terrain-as-water-when-using-20m1-basemap/
At a guess, there is a bug in the code which is exposed when a very specific construct appears in the database.
I have seen this sort of thing, years ago, in PDFs. You could create a PDF which would crash some PDF reader.
It is almost impossible to guard against this.
