Allegedly, map printers (cartographers) have errors in their
street maps to help identify blatant copies. I guess they figure
the effort to verify all of the street names, etc. makes it
unlikely that a counterfeiter would catch the deliberate error...
not sure how this has changed with more modern technology.
Monmonier says that it
continues.
The British Ordnance survey and "A to Z" street maps certainly do this.
I found one of their deliberate errors in the latest edition of our
local area map.