In addition to being a Dutch province, Friesland is also a nonexistent island south of Iceland.

Friesland started to appear on some maps during the late 1500s, as an island about the size of Ireland. This was because some Italian family (who's name i cannot remember at the moment) wanted to improve their social status. They had heard of all those explorers who traveled the seas and claimed new land in the name of their king/queen, but these exploration voyages were too dangerous and expensive for them to try. The solution was simple: They got a map over the Atlantic sea, and drew a new island there.

The funny thing is that later more and more detailed information on Friesland appeared, including drawings of the people who lived there. This is probably because sea travelers were expecting the land to be there and while they looked for it, they might have reached Iceland or Ireland.

Later on when more advanced methods for calculating positions were developed people realized that the island of Friesland was a myth, so it did not appear on any maps after the 1600s.

Source: plus some TV show on Discovery Channel