My first thought was that the this had been some sort of political statement back in 1865 but I was wrong. Due to a clerical error, the 13th Amendment to the constitution was never ratified in the state of Mississippi. CBS News:
The state’s historical oversight came to light after Mississippi resident Ranjan Batra saw the Steven Spielberg-directed film last November, the Clarion-Ledger reports.
After watching the film, which depicts the political fight to pass the 13th Amendment, Batra did some research. He learned that the amendment was ratified after three-fourths of the states backed it in December 1865. Four remaining states all eventually ratified the amendment — except for Mississippi. Mississippi voted to ratify the amendment in 1995 but failed to make it official by notifying the U.S. Archivist.
Batra spoke to another Mississippi resident, Ken Sullivan, who contacted Mississippi Secretary of State Delbert Hosemann about the oversight. Finally, on Jan. 30, Hosemann sent the Office of the Federal Register a copy of the 1995 resolution, and on Feb. 7, the Federal Register made the ratification official.