The one about the chastity belt...

A man decided to march in the holy crusades. Concluding that his wife
should wear a chastity belt while he is gone, he locks up her nether
regions and gives the key to his best friend. He tells him, "If I do
not return within four years, unlock my wife and set her free to live
a normal life."

So, the husband leaves on horseback and about a half hour later, he
sees a cloud of dust behind him. He waits for it to come closer and
sees his best friend. "What's wrong," he asks.

"You gave me the wrong key!"