In the opening sentence of Poe's classic short story "The Cask of Amontillado," Montresor claims that Fortunato had caused him a "thousand injuries" but does not go into specific detail regarding Fortunato's transgressions. Montresor's accusation seems exaggerated, and the nature of Fortunato's offenses remains ambiguous.
As the story progresses, it is revealed that Montresor is an unreliable narrator, and the audience begins to question the validity of his accusation. By portraying Montresor as an unreliable narrator, Poe raises questions regarding whether or not Fortunato's punishment is justified.
Once Montresor meets Fortunato during the carnival, he proceeds to manipulate his enemy by mentioning that he recently purchased a pipe of extremely rare Amontillado and planned on consulting Luchesi to authenticate the expensive wine. Fortunato responds by insisting that he try the wine, criticizing Luchesi's apparent expertise, and agreeing to follow Montresor into his catacombs.
It is significant that Montresor purchased a pipe because it is a large quantity, which suggests that he planned on selling a portion of it to make a profit. Fortunato is a wealthy man, and it is implied that he is interested in authenticating the wine so that he could also purchase a large quantity and turn a profit. Fortunato's motivation for purchasing a large quantity suggests that he is willing to undercut Montresor's business plan, which suggests that the "thousand injuries" were in some way related to Fortunato undermining Montresor's business deals.
Fortunato's criticism of Luchesi also suggests that he may have offended Montresor at some point in the past by publicly ridiculing him. There is also a possibility that Fortunato did nothing to harm Montresor and that Montresor is mentally unstable and acting upon his delusional thoughts.
Given the brief information in the story, one cannot draw specific conclusions regarding the exact nature of Fortunato's offenses and must take the narrator's accusations with a grain of salt.