What was Fortunato's weakness?

Montresor mentions that Fortunato's weak point is that he prides himself on "his connoisseurship in wine." Montresor uses Fortunato's pride regarding "his connoisseurship in wine" against him in order to lure him into the depths his family's catacombs, where he can bury him alive. Montresor runs into Fortunato during the carnival season and initially feigns his friendship. He then proceeds to tell Fortunato an elaborate story of how he managed to acquire a pipe of rare Amontillado wine before having an opportunity to consult him. Montresor also mentions that he is not sure if the wine is authentic or not. Knowing that Fortunato takes pride in his connoisseurship in wine, Montresor mentions that he plans on consulting Luchresi about the matter in order to make Fortunato jealous. Fortunato reveals his pride by criticizing Luchresi and insisting that he try the Amontillado in order to tell if it is authentic or simply Sherry. It is also implied that Fortunato would be interested in acquiring a pipe of Amontillado for himself while the opportunity presents itself, which is why he agrees to follow Montresor into the catacombs. Inside his family's vaults, Montresor gives Fortunato several glasses of wine to further impair his judgment before eventually shackling him to the back wall and burying him alive.

According to Montresor, Fortunato's one weakness is the pride he takes in "his connoisseurship in wine." It is this pride that Montresor plans to exploit in order to lure his nemesis, Fortunato, into his family catacombs so that he can wall the man in and guarantee his tortured and painful death.
Montresor tells Fortunato that he bought a pipe of hard to get Spanish amontillado, knowing that Fortunato will not be able to resist gloating over Montresor's mistake. Montresor uses some self-deprecating language, calling himself "silly enough to pay the full amontillado price without consulting [Fortunato] in the matter." He puts himself down and flatters Fortunato's pride in one fell swoop. Further, Montresor claims that he can tell Fortunato is busy, and so he says he will locate the other wine expert in town, Luchesi, to assist him instead. He knows full well that Fortunato will never let him do that, as he wants the opportunity to lord his expertise over Montresor. Fortunato's pride makes it easy for Montresor to manipulate him.