A case of a fake that became real.
In this case, a fictitious town that, for a while, achieved actual existence.
The town of Agloe, New York
was a "copyright trap" placed on Esso Maps during the 1930s. (That is, it was a nonexistent town whose purpose was to reveal if rival mapmakers were blindly copying the information on Esso maps.) The name was a scramble of the initials of Otto G. Lindberg (the company founder) and his assistant Ernest Alpers. They located the town at a dirt-road intersection north of Roscoe, NY.
So when the town of Agloe later appeared on a Rand McNally map, Esso accused Rand McNally of copying their map. But it turned out that Rand McNally was innocent. The town of Agloe actually had been registered with the county administration, because someone had built a general store at that dirt intersection and had named it the Agloe General Store (because that's the name they saw on the Esso map), thus bringing the town into existence.
Eventually the store went out of business, and the town of Agloe is no longer on maps. Here's the Google Map location for Roscoe, New York
Other cases of fakes that became real:
was a 1984 Usenet April Fool's Day hoax, alleging that the Soviet Union was joining Usenet. The announcement purported to come from Konstantin Chernenko, who used the email address chernenko@kremvax.UUCP. Six years later, when the Soviet Union really did link up to the internet, it adopted the domain name Kremvax in honor of the hoax.
The Annual Virginia City Camel Race
. Began as a hoax in 1959, perpetrated by the Nevada Territorial Enterprise
, but other newspapers decided to take it seriously and actually began racing camels every year in the city.
I'm sure there are other examples, but I can't think of them right now. (I'm not counting instances of names inspired by fiction, such as the space shuttle Enterprise being named after the USS Enterprise in Star Trek.)