Montresor cites a "thousand injuries" and an "insult" as his motives for murdering Fortunato. Given what you learn about the two men during the course of the story, what do you suppose the "injuries" and "insult" might be?

Really I don't think there were any intentional "injuries" or "insults". Montresor is crazy and paranoid. We can tell he is an unreliable narrator in the first few sentences of the story. The more he rambles on about his delusions of grandeur, the more the reader thinks there were no insults.