During his lunch break on May 14, 1918, William T. Robey , a bank teller at Hibbs and Company in Washington D.C., traveled, as he often did, to the post office on New York Avenue. There, he hoped to purchase a new stamp celebrating the launch of the U.S. airmail service, set to make its first official flight the following day.

The stamp was an impressive sight. It featured a Curtiss JN-4 or “ Jenny ”, the same plane set to deliver the mail the following day, and was printed in carmine rose and deep blue. The striking color scheme no doubt wooed buyers, but like many of the avid collectors who gathered at post offices in Philadelphia, New York and the nation’s capital, Robey also knew that it enabled an even more spectacular possibility—a printing error. It was just the second time the Postal Service had attempted a two-color stamp and with the fervor of World War I, sloppy mistakes were a more likely occurrence.

Among the many philatelists, Robey was the lucky one. Instead of flying high through the skies, the Jenny on his stamp appeared upside down, as if it were doing an elaborate aerial flip for some grand barnstorming performance. Even luckier for Robey, the person selling him the stamps on that fateful day had never seen an airplane and couldn’t tell the difference . When he saw the error, Robey saw opportunity, and he coolly asked to purchase a 100-count sheet for $24.

Soon after, he sent word of the mistake to fellow friends and collectors, and it didn’t take long for the news to spread to postal inspectors, who were eager to reclaim the errant stamps. Of course, Robey rebuffed their offers, and for a few days, he hid the sheet of stamps under the mattress in a one-room apartment he shared with his wife. Under mounting scrutiny, he was eager to make a deal, and in a panic, he sold the stamps to Eugene Klein, a Philadelphia businessman an avid philatelist for $15,000.

The money allowed the Robeys to purchase a new house along with a car, which as the story goes, William promptly drove through the back wall of his garage. Its symbolic of a much larger blunder that emerged from his panicked selling: Robey assumed that more flawed stamps would emerge since they were typically printed on a larger 400-subject plate. But the other mistakes were caught and destroyed. Had Robey been patient, he could have made even more.

Klein quickly sold the sheet to his friend, Edward Green, and made a sizeable profit on the deal. By now, the so-called “Inverted Jennies” were growing in notoriety. As one writer notes in his retelling of the events they “blossomed into the Taj Mahal of stamps, the Fort Knox of collecting, the Mona Lisa of timbromanie and the Holy Grail of philately.”

Klein convinced Green, the son of the notorious and parsimonious “Witch of Wall Street”, to divide up the original sheet and number the backs of each stamp in order to keep a record of their ownership. Green created one block of eight stamps, seven blocks of four stamps and 64 individual stamps with various perforations depending on their location. Green kept the best examples for himself, and sold the remaining ones for between $175 and $250.

While the prices of the stamps continued to rise, Green’s remaining stamps became the focus of one particularly unbelievable philatelic legend. At a stamp collectors club, he allegedly threatened to burn all the straight-edged stamps and was only dissuaded by the horror of his fellow collectors, who implored him to cease. From there, he eventually placed the remaining straight-edge stamps in a safe to limit supply, where they remained until his death in 1936.

When collectors rediscovered the stamps, they were dismayed: Over the years, the Inverted Jennies had become stuck together, perhaps because they were left outside during one of Green’s many yacht escapades. To unstick the fused stamps, an auction house had to use water to remove the gum before separating them with a ruler. One of these gumless stamps is on display at the “ Gems of American Philately ” exhibition at the Smithsonian’s National Postal Museum along with a four-stamp block and two more Inverted Jennys.

And, you might wonder, what happened to the remaining stamp blocks? Most have found their way to wealthy stamp collectors—an anonymous buyer purchased one for more than $1 million in September 1989; at a more recent auction in New York, a different block sold for nearly $3 million. The enormous price comes from a couple of factors according to Daniel Piazza, a curator at the Postal Museum.

“It’s the romance of early aviation, it’s the dramatic image of the plane flying upside down, it’s the red white and blue colors,” Piazza says. “It just has so much going for it: The end of World War I, the beginning of civilian aviation, carrying the mail by the air.”

As Piazza points out, stamp collectors don’t actually consider the Inverted Jennys to be particularly rare–100 stamps is a sizeable amount within the world of philatelists. Still, many people wanted them, and as with most valuable items, the Inverted Jenny also attracted no shortage of criminal interest. The most high-profile heist, in which a block of four stamps was stolen during a Philatelic Society convention in September 1955, remains unsolved some 60 years later, though three of the four stamps have been recovered. Depending on its condition, that one stamp is worth a small fortune–a single Inverted Jenny has sold at auction for more than $500,000 in recent years.

Even the Postal Service has tried to capitalize on the popularity of Inverted Jennys. In 2013, they reissued the famous stamp, selling them for a modest $2 each. In a humorous spin, though, they decided to print the stamps upside down on purpose–but also created 100 sheets with the airplane flying right side up in an attempt to generate publicity and stir interest in collectors.

But the upside-down visage actually proved somewhat prescient. Piazza says that the plane engraved on the Inverted Jenny–Number 38262, which flew from Potomac Park near Washington, D.C. on May 15, 1918 for the first airmail delivery–was piloted by an inexperienced man, who got lost during his flight and had to make a crash landing. When his plane hit the soft ground in a field in rural Maryland, it flipped over. Art, as it so often does, imitated life itself. And as Piazza points out, the story continues to dazzle many years later for its sheer inconceivability.

“People, when they step up to that stamp, intuitively understand,” he says. “On a very visceral level they understand why the stamp is treasured–they can understand why someone would want to own one.”