Fortunato did not do anything to insult Montresor.
There is no specific insult to Montresor mentioned, but the way that Montresor describes Fortunato’s insult makes it seem like it was either very slight or nonexistent.
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge. You, who so well know the nature of my soul, will not suppose, however, that gave utterance to a threat. At length I would be avenged …
Fortunato clearly has no idea what he is doing to annoy Montresor. Montresor is convinced that Fortunato has done him horrible wrongs, but these are all in his head. Montresor is not in his right mind, and so he has imagined that Fortunato has insulted him.
You can tell that Fortunato isn’t aware that Montresor considers him an enemy by the way he greets him.
It was about dusk, one evening during the supreme madness of the carnival season, that I encountered my friend. He accosted me with excessive warmth, for he had been drinking much. … I was so pleased to see him that I thought I should never have done wringing his hand.
Yes, Fortunato has been drinking and so his judgement might be impaired, but why would he be so happy to see Montresor if he spent so much time insulting him? Why would Montresor be happy to see him? Actually, Montresor is happy to see him because he has been planning his murder. It makes no sense though, that if Fortunato was really an enemy and had really done all of these horrible wrongs to Montresor that he would so willingly go with him.
The insults, or the thousand injuries, are all in Montresor's head. They are the product of a delusional mind. Montresor is a madman, and while madmen might make entertaining narrators, they do not make reliable ones. They do, however, make excellent murderers. Montresor carefully plots Fortunato's murder because he is convinced he has done him some horrible wrong.