Montresor never makes specific what Fortunato has done to him, suggesting that the "thousand injuries" he may or may not have suffered are less important than Montresor's perception of injury.
We do get a hint that Fortunato considers himself superior to Montresor. When Montresor says he is a mason, Fortunato takes this to mean a Freemason, a group which apparently Fortunato is a part of--and Forunato exclaims "You! Impossible!" He "recoils" when Montresor shows him the trowel, and says "You jest." Then his mind returns to his quest for the Amontillado, and he urges Montresor on.
This dialogue, though short and merely suggestive, speaks volumes in a compact story. It hints that Fortunato considers that he is lowering himself to be with Montresor, a man he cannot seriously consider a fellow Mason, and only condescends to do so because of the bait of the rare wine.
On the other hand, Fortunato's snobbery is not stated directly as fact by either of the characters. One of the most interesting facets of this story is its porousness, the way it leaves itself open to construction and interpretation.