The dangers of excessive politeness are perfectly exemplified in the medieval legend of Parzival, Arthur's purest knight. His quest for the Holy Grail leads him to a castle where he is welcomed by a wounded lord. At dinner, a mysterious bowl captivates Parzival's attention. He's dying to know more about it, but he holds his tongue. His training as a knight has taught him that it's uncourteous to express too much curiosity.
Tragically, he doesn't realize that he has arrived at the very place where his quest could be satisfied. The wounded lord is actually the fisher king, the marvelous bowl is the Grail, and he is being presented with a magical test. The test consists of a simple task: to ask about the bowl. Because Parzival fails to do so, the king doesn't reveal the secret and doesn't give him the Grail.
The next morning, Parzival wakes up to find the castle empty, and he leaves having missed the very opportunity he wanted most.