The Northwestern Territories of the United States - From Dr. R. Brooke's General Gazetteer Improved, Johnson & Warner, 1812
There's a curious feature on old maps of Michigan and the Northwest Territory: a large plateau in the central Lower Peninsula with a ridge of mountains extending south through Indiana and Illinois. According to the Philadelphia Print Shop, where I downloaded these scans, the feature was first found in maps of the area from the late seventeenth century.
The Upper Territories of the United States - From Carey's General Atlas Mathew Carey, 1814
It's curious that the error remained in maps for over a century.