As the German army invaded Belgium during World War I, a rumor arose among the Allied Powers that German soldiers had brutally mutilated a Belgian baby. Many people had claimed to have seen the baby, the press reported the event as fact, and it soon became conventional wisdom.
Historians now recognize this “event” as mythological.
There is no solid or reliable evidence that there ever was such a baby, but it was a horrendous story that everybody who was already opposed to the Central Powers seemed to want to believe. This is a prime example of a phenomenon seen elsewhere: if a belief fits into a framework or narrative that is strongly held about an emotionally-charged issue, this belief takes on a very sophisticated life of its own.