We have a professional cartographer, who travels, creates maps, duplicates them, and sells them to people as he travels. Unfortunately, he is not the best at his job - and he has nothing other than a compass, paper, and quill (and ink).
I would like his mistakes to be somewhat meaningful -
"meaningful" defined as: mistakes which would cost a traveler time in figuring out there was a mistake and in having to figure out the correct action in order to get to where they were intending to go
What accidental (or on-purpose, but he still intends the map to be useful to anyone who may use it) mistakes would most affect a traveler who is trying to use the map, yet still seem correct to people who are vaguely familiar with the area?
As suggested in the meta question, the best answer will be the method that is most wrong and least obvious to inhabitants.