24c Curtiss Jenny inverted block of four
One of the most iconic and recognizable stamp errors ever discovered is the 1918 24c "Inverted Jenny" air post stamp. In 1918 the Post Office issued its first air mail postage stamp, to promote the newly established air mail service. A special 24c stamp was prepared, depicting the "Curtiss Jenny" biplane in the center in blue, surrounded by a red frame. One sheet of 100 stamps was discovered with all of the stamps within the sheet having had the blue biplane in the center printed upside down.