An inverted Jenny plate block of four (note that the blue plate number is inverted as well).
The World's First Airmail Stamp - the 24¢ Curtiss Jenny biplane is one of the most widely-talked about postage stamps in the world. It was made forever famous when a single sheet of 100 was discovered in 1918 with the blue airplane inverted. The mistake made these stamps worth six figures to collectors today. Interisting too, the stamps history is almost as twisted as the event it celebrated.
The inaugural flight of the U.S. Air Mail Service was scheduled to begin at the Polo Grounds in Washington D.C. and run to New York with a stop in Philadelphia on the way. On May 15, 1918, as President Woodrow Wilson and other dignitaries looked on, the engine in the Curtiss Jenny failed to start because of an empty fuel tank. Once the plane and its cargo of 140 pounds of letters finally got airborne, the pilot George "Wrong Way" Boyle became lost and flew 25 miles in the wrong direction. He was forced to make an crash landing in an empty field. After this, the plane was non-flyable, and its mail cargo was delivered to its final destination by truck, 2 days late!