The "Inverted Jenny" is a misprinted U.S. postage stamp showing an inverted image of a blue airplane. The error occurred on the 24-cent airmail stamp of 1918. Only one sheet of one hundred inverted center stamps was sold, and no other examples have been discovered.
How the Inverted Jenny, a 24-Cent Stamp, Came to Be Worth a Fortune
(Smithsonian Magazine, May 5, 2018)
How an upside-down biplane on a 24-cent stamp, at the Smithsonian’s National Postal Museum, seemed to jinx early attempts at carrying the mail by air.
The Inverted Jenny
(Smithsonian Magazine, June 30, 1996)
Mark the centennial of an epic mistake at the National Postal Museum where several of these world-famous stamps are on view.