What does Montresor mean when he says that Fortunado has a weak point in "The Cask of Amontillado"?
"He had a weak point—this Fortunato—although in other regards he was a man to be respected and even feared.
He prided himself on his connoisseur-ship in wine." (enotes etext p. 4)
Montresor wants to take advantage of Fortunado’s pride in his knowledge of wine in order to trick him to go into the catacombs with him.
Montresor is convinced that Fortunado has insulted him in some way.
THE THOUSAND INJURIES of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge. (p. 4)
Although it is not clear what the insult was, we can gather that Fortunado has no idea that he insulted Montresor, or that it was such a big deal. He still thinks that Montresor is a friend. Nonetheless, Montresor has been insulted and develops a cunning plan to trap Fortunado. He is going to take advantage of his love for wine.
In painting and gemmary, Fortunato, like his countrymen, was a quack, but in the matter of old wines he was sincere. (p. 4)
Montresor knows that he can get Fortunado’s attention, even during the carnival, by telling him he has a rare wine. He can get him to follow him down into the catacombs, and once there they will be alone. Then he can kill him.
The theme of revenge is a thread throughout the story. Montresor feels that whatever small insult Fortunado did to him is worth his life. Yet Montresor is not completely insane. He is wily enough and sane enough to use Fortunado's love of wine against him.