A short story by
Jorge Luis Borges.
The story is based upon the same idea as The NodeShell World Fiction Project. A group of shadowy conspirators decide to make up an entire country by writing bogus entries in an encyclopedia, thinking it a high joke. As they continue to embellish on the mythical culture of Tlon, they derive much enjoyment out of watching credulous academics and geographers refer to Tlon as though it were fact, without blinking an eye.
That is, until knowledge of Tlon starts to enter popular culture. And then more stories of Tlon appear -- tourist guides, anthropology studies things which were never written by the conspirators.
And then real Tlon currency is found...