Estimated value: US$3 million
The 24-cent Jenny stamp was the first stamp for air-delivery issued in 1918. The stamps were printed on sheets of 100, but each sheet had to be fed through the printing machine twice. Therefore, it was very easy to accidently flip the sheet on the second pass, and this is exactly what happened. One of these sheets was printed with the jenny plane upside down, and sold before the error was noticed. This was how the Inverted Jenny got created.
Only 100 pieces of the Inverted Jenny can be found in the world nowadays. A block of 4 stamps was purchased in 2005, for US$3 million.