Debatably the rarest stamp error in U.S. history, the Inverted Jenny is among the most mythical. The plane depicted on the stamp is the JN-4HM, built by the Curtiss company in the middle of World War I (95 percent of U.S. pilots trained on JN-4s during WWI). Philately, like many other hobbies, enjoys the self-referential: this was the first plane used to deliver mail. A printing error caused the blue vignette—the airplane and the air around it—to be printed upside down, while the red border framing the scene was printed correctly. The error only appeared on a single sheet of 100 stamps, which has since been broken up, so that mostly single examples of the stamp exist, though there remain two blocks of four. In 2016, a single Inverted Jenny sold at auction for $1,351,250.

The Jennies—military biplanes—were modified for government airmail service with extra fuel tanks, a different engine, and a hopper for mail. They often crashed. In fact, the very first U.S. Post Office Department airmail flight on May 15, 1918 ended in disaster. The pilot flew in the wrong direction and crashed in a farmer’s field, ironically next to a property owned by Otto Praeger, the postmaster official in charge of airmail. “None of the first day’s mail made it,” says Scott Trepel, president of Siegel Auction House. “They had to send it the next day.”