A Washington clerk walked into a post office during his lunch break on May 14, 1918, and bought something that appeared ordinary: a sheet of 100 new 24-cent airmail stamps. Then William Robey noticed something was wrong. Every Curtiss JN-4 biplane printed on the sheet was upside down.
What followed turned the sheet into one of the most famous errors in stamp-collecting history — the Inverted Jenny. More than a century later, individual stamps from that sheet have become multimillion-dollar collectibles.