The Cask of Amontillado" is a story of revenge, but the reader is never told exactly what Fortunato did to warrant such vengeance.
In fact, throughout the story, the reader gradually realizes that Montresor is an unreliable narrator; that whatever insult Montresor believes Fortunato committed is probably imagined or exaggerated. It's certain that Fortunato has no idea of Montresor's anger, and this makes the story even more tragic and frightening. The seemingly happy jangling of the bells on the top of Fortunato's cap become more and more sad the deeper the two venture into the catacombs.