The Inverted Jenny is a US postage stamp first issued in 1918. The stamp features an inverted or upside down Curtiss Jenny airplane. The inversion of the plane was an accident and as a result only 100 were produced, making this one of the most rare stamps in existence. An Inverted Jenny stamp was sold in 2007 for almost 1 million dollars!
During the 1910s, the USPS was experimenting with air mail delivery. The Curtiss Jenny was the plane chosen for this task and the stamp was introduced with a price of 24 cents. The cost of regular mail delivery at the time was 3 cents, so this was a very large increase. The USPS wanted to get this service running very quickly and as such only gave a week between the beginning on the engraving process and production. This rush resulted in the Inverted Jenny.