I may be way off base here, but I have always understood that deliberately making an error is an homage to God or the gods, in effect saying that the maker of the item is not worthy to rank with the perfection that is God's/gods' alone.
Have also heard that making something perfect would inflame the ancient gods and bring disaster when they wreaked their anger upon the person for the impertinence.
In Chinese and Japanese brush painting a small error is generally found especially in the older masterpieces.
I liked the Native American idea of letting the evil escape mentioned above.
Oh, yes, one last thing ... some maps have a non-existant town, usually on a third class country road (unpaved) and the "town" is the name of the map-creator ... it's an easy way for a cartographer to prove she/he did the map.
