The rarest U.S. stamp ever printed is one with an error on it. The Inverted Jenny, which was issued in 1918, had a face value of just 24 cents. However, today the stamp is valued at $1.35 million. The high value of the stamp is determined by the mistake in printing the airplane upside down. The clerk in Washington DC who sold over 100 of these stamps had never seen a plane and sold these stamps without seeing the error. With its recent auction in 2016, the Inverted Jenny is one of the most valuable stamps worldwide.