The best example of this that comes readily to mind is the British government's propaganda effort saying that World War 2 fighter pilots were eating carrots to improve their night vision. The truth, as we now know, was that it was the invention of radar - a technology that we gave to the Americans.

Interesting. And we all know that british people do love carrots. Is a sunday roast complete without them?:-)

I hate to quibble, but the following url indicates that it was the Germans who first made the breakthrough:

https://en.wikipedia.org/wiki/History_of_radar

However, it was not until the early 20th century that systems able to use these principles were becoming widely available, and it was German inventor Christian Hülsmeyer who first used them to build a simple ship detection device intended to help avoid collisions in fog (Reichspatent Nr. 165546). Numerous similar systems, which provided directional information to objects over short ranges, were developed over the next two decades.