In my opinion, the initial incident in this story is not really even seen in the story. To me, the initial incident is whatever Fortunato did in the past to Montresor.
The initial incident is the thing that leads to the rising action in the story. To me, the rising action in this story is the interaction between Montresor and Fortunato as Montresor maneuvers Fortunato into going down into Montresor's wine cellars.
So the initial incident has to be whatever caused this to happen. In my opinion, that is the conflict between them that is only referred to in this story.
I agree with the previous post that the initial incident in the short story, "The Cask of Amontillado," is the offense that Fortunato commited against Montressor. Unfortunately, Montressor never reveals what Fortunato has done to him to cause the vengeful incidents that follow. We only know what the first lines of the story tell us.
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.
It appears that Montressor was unhappy with Fortunato's longtime treatment of him ("The thousand injuries"), but the last straw was the unnamed insult.