With my rudimentary knowledge of land navigation, I know that north on a map, magnetic north, and true north are often a few degrees different - enough to get you lost if you're traveling long distances. I understand this to be caused by the curve of the planet and the actual location of the magnetic poles, but if I believed the earth was flat, wouldn't I get lost a lot?
It's difficult to answer this, given that no-one who lived in a period where they were doing anything like accurate cartography believed in a flat earth. The idea that people in the Middle Ages believed in a flat earth is a myth invented in the nineteenth century - see Jeffrey Burton Russell's Inventing the Flat Earth: Columbus and Modern Historians (1991)
So early cartographers were well aware that the earth was round and never had to deal with the problems involved with trying to make maps on the assumption it was flat.
FYI, more info on flat Earth in the FAQ: