Here's a new take on Fortunato's "weakness." Almost everyone thinks it's his overweening pride in his ability to judge wines and this true to a degree, but let's dig a little deeper.
"The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge." Is this particular insult that seemed to push Montressor over the edge clearly stated? No! It must searched out. Once Montressor has Fortunato well into the catacombs and has plied him with wine, the nature of the insult begins to reveal itself.
Fortunato empties a flagon of wine and "threw the bottle upwards with a gesticulation I (Montressor) did not understand." He repeats the gesture. Fortunato asks him if he comprehends the motion. Montressor does not and Fortunato says, "Then you are not of the brotherhood," a reference to The Order of Masons. Montressor insist that he is, but Fortunato responds, "You! Impossible! A Mason? Montressor replies that he is and produces a trowel, the symbol and tool of masonry. Fortunato takes it as a joke.
It is my contention that the insult delivered by Fortunato to Montressor was probably blackballing him or somehow keeping him out of the Masons. The exquisite irony of Montressor's "masonry" revenge, the walling up of Fortunato, is completely lost on Fortunato. he doesn't get it and probably doesn't even remember what he had done. You can assume that Montressor is mad, but it's the madness of a genius.
Luckily for Montressor, his "friend" Fortunato had several weaknesses, but his obvious love of the grape was his biggest. Montressor tells the reader that Fortunato "prided himself on his connoisseurship in wine," and Montressor shows genuine respect for his knowledge. Montressor uses this weakness to lure Fortunato into his trap, claiming that he has a rare vintage of Amontillado--a Spanish sherry--for him to sample. The Amontillado is only a ruse--no bottle is ever produced--and once Fortunato enters Montressor's cellars, he never returns.
Fortunato's weakness was his weakness to boast about his knowledge of vintage wines:
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 connoisseurship in wine. Few Italians have the true virtuoso spirit. For the most part their enthusiasm is adopted to suit the time and opportunity to practise imposture upon the British and Austrian MILLIONAIRES. In painting and gemmary, Fortunato, like his countrymen , was a quack, but in the matter of old wines he was sincere. In this respect I did not differ from him materially; I was skilful in the Italian vintages myself, and bought largely whenever I could.
It is this which proves to be his hubris. Montresor tells him that he wants his opinion on the genuineness of "a pipe of what passes for Amontillado." The convivial atmosphere of the carnival and the excessive drinking - "The gait of my friend was unsteady, and the bells upon his cap jingled as he strode" - blind Fortunato to Montresor's evil intentions and he is lured easily into Montresor's vaults where he is buried alive.