I remember reading this joke online a long time ago. A knight was about to leave home for a crusade with the rest of his round table and told his best friend, "I want you to keep this key safe, it belongs to my wife's chastity belt, so guard it with your life. Keep her safe until I return."
As the knight rode off on his horse up the hill, the friend came running and shouting to the knight, so the knight stopped and asked, "what's wrong?"
The friend said, "You gave me the wrong key, it doesn't work on the belt!"