The Republic of Cospaia, situated in northern Umbria, Italy, existed independently from 1440 to 1826
The Republic of Cospaia, situated in northern Umbria, Italy, existed independently from 1440 to 1826

en.wikipedia.org
Republic of Cospaia - Wikipedia

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.