Sir Galahad goes over the Sir Lancelot's house and says "Well, I'm off to the Crusades. And as you are my most faithful friend, I will entrust you with the Key."
Lancelot asks "What's the key to?" and is told "This is the key to my wife's chastity belt. I could entrust it to no other knight than you."
So Galahad gets on his horse and gallops off in the general direction of Jerusalem. But after an hour he pauses to check his directions, and sees a cloud of dust behind him.
It turns out to be Sir Lancelot galloping up. "Hey Galahad! he cries. "Wrong key!"