What was the cause of Fortunato's fate?

The narrator in "The Cask of Amontillado" is unreliable, narrating his tale in great (and rather frenzied) detail fifty years after the act takes place. He opens the story with these lines:

The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.

Fortunato meets his fate, then, through some unnamed "injuries" that he has seemingly inflicted upon Montresor—or at least, Montresor perceives it that way. At some point, these injuries became personal, insulting. When Fortunato crossed this line, Montresor decided that he must kill him.
Has Fortunato truly insulted him? Has our unreliable narrator imagined the insult? Does the insult warrant death? Does the insult even matter? Poe doesn't write these details into the story, leaving readers to conclude whether the death Fortunato meets is a deserved one. Montresor takes this man, who is "respected and even feared," uses his weak point of a pride in wine connoisseurship, and lures him to his death for whatever perceived crimes he has committed against our narrator.