I Have Only Just Found Out,

that one of the most famous paintings in the White House is a copy,

of an original painting that is in the Smithsonian, how can you tell the copy in the White House from the original?

  because a misspelling was intentional,

the Lansdowne portrait was completed in the fall of 1796 by American artist Gilbert Stuart, who then went on to make several copies, the White House's copy of the Lansdowne portrait. "UNITED STATES" is misspelled as "UNITED SATES" to mark the copy.