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.