On the map of Europe, Poland has a strange nonexistent river flowing through it. It enters the Baltic Sea more or less where the Vistula river really does. But going south it mysteriously veers east into Belarus.

I know sometimes cartographers leave nonexistent landmarks on their maps to catch plagiarists blindly copying them. Not sure if that's something similar here.

Could also be a method of identifying information leaks. Leave unique imaginary landmarks on different prints associated with certain people or groups of people that received that print.

Or just wrong info. Who knows.