Inverted Jenny

Definition: A Curtis Jenny JN-4H is seen flying upside down, or inverted in relation to the stamp frame, on this classic U.S. air mail error rarity. The error was created in 1918 when the sheet of 100 two-color stamps passed through the press upside down for its second impression. Value of a single example as of the beginning of the 21st century is around three quarters of a million dollars, based on auction sales.