The intriguing tale of a 1918 airmail stamp (actually, the first airmail stamp) misprint - The Inverted Jenny:
Most printing errors involve either an inverted plate or a sheet fed improperly to the printer. And most are caught quickly, either by a print inspector or a postal clerk. But the clerk who sold that sheet to Robey didn't spot the mistake. Asked about it later, he replied, "How was I to know the thing was upside down? I never saw an airplane before."