What was Fortunato's weak point?

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.