There may be no definite answer as to how Fortunado wronged Montressor. However, by conecting symbols, a highly educated guess can be made. Based on the interconnection of the book's symbols, how might Fortunado have wronged Montressor?
I agree that it was some kind of insult. I honestly don't think that Fortunado would have gone down into the crypt with Montresor if he had been having an affair with his wife. I personally think that the hint is in Montresor’s craziness. He did brick the man up! I think it was some slight, insignificant thing that Fortunado did not even remember.
I don't agree with #2 & #3 that there is reason to believe that Fortunado had an affair with Montressor's wife. I don't remember any indication of this. I do, however, recall that on the way down to the catacombs, Montressor speaks of his family's coat of arms. He refers to the Latin motto "Nemo me impune lacessit" meaning "No one INSULTS me without impunity."
The coat of arms and the word insults are key to understanding why Montressor is seeking revenge on Fortunato. The family name meant everything to social standing and reputation. People guarded their family name because it represented exactly who they were and what they stood for. Because Montressor mentions these things, I believe that Fortunato made an insult against Montressor's family name. It doesn't matter what the insult is; it only matters that there was an insult.
I have to agree with #2 on this one - what "highly educated guess" are you making? I think actually part of what Poe was trying to do with this story was present us with a profoundly unreliable narrator who didn't have a wrong against Fortunato and was just incredibly disturbed - as is displayed through his speech and actions. For Montresor to have an actual motive for his crime would lesson the terror and horror that Poe was trying to achieve greatly.
I suspect you're getting at a school of thought which suggests that Fortunato had an illicit affair with Montressor's wife. I'm afraid I just don't see it, but I'd be interested in hearing your reasoning.
symbol is the catacombs...it means death and the rare amontillado which means too much desire of revenge..