The residents of Burnsville Township, Minnesota, have changed the name of their newly incorporated city to "Nowthen"—the name mistakenly given to its first post office.
During the 1890s, a community leader created a list of possible names for the post office and wrote, "Nowthen, one of these ought to do."
Historians say the post office came to be known as Nowthen. [Link]
[Thanks, Nancy!]
I think something similar explains the name of Uncertain, Texas, but, of course, nobody's really sure.