The Republic of Cospaia, situated in northern Umbria, Italy, existed independently from 1440 to 1826
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Republic of Cospaia - Wikipedia
https://en.wikipedia.org/wiki/Republic_of_Cospaia
Its unexpected independence resulted from a boundary error in a land sale by Pope Eugene IV to the Republic of Florence. Due to confusion over two streams named "Rio" a strip of land became terra nullius.