The 24 cent Jenny Stamp was created for delivery of mail by air – a new concept in 1918. This is a large sum compared to the 3 cents that it costs for ground delivery. As a result, the stamp was created and printed in a hurry.
The Jenny Stamps were printed in sheets of 100, but each sheet had to be fed through the printing machine twice. Therefore, it’s very easy to accidently flip the sheet on the second pass, and this is exactly what happened. One of these sheets were printed with the jenny plane upside down, and sold before it was noticed, creating the Inverted Jenny error.
The sheet of stamps were subsequently sold for ever higher amounts. Eventually, the sheet was broken up when the owner realized that the Inverted Jenny Stamps were worth more (resale wise) when separated.
A block of 4 stamps was purchased in 2005 for $2.9 million dollars.
The Inverted Jenny is probably the stamp with the most famous error in American philately. This stamp was first issued in 1918. The Curtiss JN-4 aeroplane which is in the center of the stamp is upside down. In November 2007 one of these stamps was sold at auction for $977.500. A month later a mint never hinged example of this stamp was sold for $825.000. In 2005 a block of four of these stamps was sold for $2.9 million. Although prices dropped for a while, in 2016 a very well-centred example of this stamp was sold for $1.35 million.