In 1818, mathematicians George Camak and James Gaines were tasked with marking the border between Georgia and Tennessee. This was very difficult in an age long before GPS was invented. Atlas Obscura explains:
Without any modern tools or the zenith sector he requested at his disposal, Camak was forced to rely on inaccurate astronomical tables and the stars. Without those tools, Camak marked the 35th parallel about one mile south of its actual location.
A mile might not mean much, but a similar surveying error once resulted in the US building an expensive fort inside the borders of British Canada. Camak suspected that his astronomical tables were inaccurate, so he returned to the task eight years later and moved the marker.
That survey, though, was also wrong, and Camak's work resulted in a border dispute between the three states covering 68 square miles of land:
Over the years, several disputes have broken out among legislatures from Georgia and Tennessee over access to water in the Tennessee River. In 2007, the original stone maker was mysteriously stolen, it has since been replaced, however, it’s unclear what happened to the original marker .
You can visit the stone by following the directions provided by Atlas Obscura.