It tends more to be things which haven't been mapped yet rather than inaccuracies as such. Most mappers have concentrated on mapping their local town or city rather than national borders. See Kiev (https://www.openhistoricalmap.org/#map=16/50.4633/30.5218&la...) and London (https://www.openhistoricalmap.org/#map=15/51.5163/-0.0900&la...) for example. While the project may have been going a while, it was only last year that the time slider was introduced which made it properly functional as a historical map. At the moment there are still few mappers (including myself) but I get the impression that OHM is now beginning to develop more quickly!