Answer: North Dakota

On November 2, 1889, North Dakota became the 39th state of the United States of America by meeting the requirements for statehood combined with the approval of then U.S. President Benjamin Harrison. For the next century, things went rather smoothly and certainly no one even questioned if North Dakota had met every requirement for statehood or not.

No one, that is, until historian John Rolczynski discovered a small error in North Dakota’s state constitution while writing a book about the state. Specifically, thanks to the omission of the word “executive” from a single sentence, there was no constitutional requirements for members of the state’s executive branch to take an oath of office. This omission conflicts with federal requirements for statehood and thus, technically, North Dakota was never legitimately a state.

Obviously North Dakota has been treated like a state for well over a century now, but that didn’t stop Rolczynski from doing his best to get his state’s constitution fixed so that they weren’t, no matter how small the technicality, left out of the Union. He discovered the error in 1995 while researching his aforementioned book, and after insisting long enough that somebody ought to fix it, somebody did. In 2011, North Dakota senator Tim Mathern introduced an amendment to fix the minor (but important) error in the state constitution, and in the November 2012 election the citizens of North Dakota voted in favor of LRCA Measure 2 which set the requirement for the governor and all executive branch officials to take a formal oath of office (thus amending the constitution and removing the technical loophole therein).