History, intrigue and a misprint combine so that a single stamp has sold for $2 million at auction. It is a U.S. postage stamp from 1918. But this stamp's got lore. The design – which normally shows the "Jenny" Curtiss biplane the right way up – was already important because it was used on the stamps for the world's first regularly-scheduled government airmail service. What makes this particular stamp noteworthy is that in the glory-induced rush of stamp making at the time, the workers who were printing this sheet accidentally placed the Jenny upside down. The single sheet of 100 so-called "Inverted Jennys" was sold before anyone caught the mistake, and they have become treasured collector items ever since. It is the "icon of stamp collecting," according to Scott Trepel, the president of Siegel Auction Galleries in New York and an expert in the stamp field. He says to keep in mind that planes weren't particularly common in 1918: "People weren't familiar with what they looked like, and so the inverted plane on the stamp slipped through the inspectors, slipped through the clerk at the post office. And even he said, you know, 'Look, don't blame me. I don't know what a plane looks like, so I didn't recognise it when I sold it.'" Trepel says this one is extra special because it's in really good condition after being in storage for decades: "It never was exposed to light. The colours were beautiful. The paper was bright. The back of the stamp, the gum had never been hinged and put into an album."