Edgar Allan Poe, in his short story The Cask of Amontillado, does not specify the precise nature of the "inciting incident" that drives the story's narrator, Montresor, to seek the demise of his intended victim. The story's opening sentence is purposely vague with respect to this matter:
"THE thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge."
We can only surmise from this comment that Fortunato has verbally insulted, or criticized to unspecified third parties, the story's narrator. Certainly, Fortunato is presented as a pompous blowhard, thoroughly arrogant, particularly on the subject of wine, the bait employed by Montresor to entrap his nemesis. At one point, continuing to lead the inebriated target of his wrath deeper into his cellar, Montresor begins to explain that another colleague or friend, Luchresi, has attested to the identity of Montresor's cask of wine, prompting Fortunato to sharply respond, "He is an ignoramus." All we know of Fortunato, therefore, is that he is an arrogant ass, given to condescending remarks directed towards others, and it was apparently a history of such comments uttered towards or about Montresor that constituted the "inciting incident."
The inciting incident happens with a verbal exchange. The exchange probably an insult however, is not what caused Montresor to seek revenge, it was like the straw that broke the camel back.
The last indignity Montresor could suffer at the hands of Fortunato. The opening quote "THE thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge." shows that Montresor feels that he has suffered indignities from Fortunato time and time again.