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.
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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.
North Korea is a closed country with limited access to information (think before the internet). On the flip side, I don't think catching copyright violators would be high on the list of concerns of the people making these maps.