Is there a more famous misprint in the history of U.S. philately? In 1918, the Post Office decided to introduce a new Air Mail rate at 24 cents; a big jump from the 3 cents that it cost to send a regular letter at the time. There was a fervor at the printing office to get the new stamps out in a hurry. That rush, paired with the two plate printing process (the plane is printed first in blue and then the frame is printed in red), gave rise to the error. Turns out, the plane is printed right-side up; it's the frame that is upside-down. Only 100 of the 1918 Inverted Jenny stamps exist.
This release, honoring that historical slip-up, is being released with great fan-fare at the William H. Gross Stamp Gallery opening at the National Postal Museum this Sunday. In 2005, Mr. Gross had purchased a block of the Inverted Jennys at an auction which he then traded for a 1cent Z Grill. This gave William Gross the only complete collection of U.S. 19th century stamps. No wonder he's getting a gallery at the NPM, right?