Carrots

British gunners in WWII were able to locate and shoot down German planes at night due to the invention of radar, which the Germans knew nothing about.  To cover up the invention and extreme effectiveness of radar, the British spread about an urban legend that said that they massively increased the night vision of their pilots by having them consume large amounts of carrots.  This lie not only convinced the Germans, but also had a bonus effect of causing many British people to start planting their own vegetable gardens, including planting carrots.  This urban legend has persisted even to this day.