What are the "thousand injuries of Fortunato"?

This is such an interesting question because in the very first sentence, the narrator tells us, "The thousand injuries of Fortunato I had borne as best I could, but when he ventured upon insult I vowed revenge."
The narrator is trying to convince us from the very beginning that Fortunato has wronged him so unjustly that Fortunato deserves death. That is a pretty strong position to take, so where is the support for these "thousand injuries"?
Interestingly, the narrator doesn't really develop those details. And that is part of the problem, because this narrator has many qualities that lead us to believe he is unreliable and that maybe the details he is providing aren't exactly factual. (Poe presents similar narrators in "The Pit and the Pendulum" and "The Tell-Tale Heart.")
In the second paragraph, the narrator tells us that "it must be understood that neither by word or deed had I given Fortunato cause to doubt my good will." In effect, the narrator is therefore bragging about how well he was able to deceive Fortunato into trusting him on the walk that led to Fortunato's death. If he so effectively deceived his victim, is he equally talented at deceiving the reader?
And it is also important that in the final lines, the narrator lets us know that it has been "half a century" since this murder took place. So how clear are his memories of these "thousand injuries," anyway?
It is possible that Fortunato inflicted no injuries on our unreliable narrator at all. Surely those would have been worth recounting as justification for the murder that transpired, so the absence of these key details really builds a case that Fortunato may not have been to blame for his untimely end.

Many readers have complained about the fact that Montresor never offers any examples of the thousand injuries he claims to have suffered. Some readers think Montresor must be insane and only imagining the injuries. The story was originally published in 1846, and to this day the thousand injuries have remained a puzzle. It almost seems as if Edgar Allan Poe was playing a joke on his readers, leaving them to wonder what those injuries could have been or whether there were any injuries at all. But Montresor is not addressing us readers; he is addressing a confidential confession to someone he calls "You, who so well know the nature of my soul." That person must have known about some of Montresor's grievances, but he or she is dead now, and nobody else will ever know what any of those grievances were. They must be kept secret, just as Fortunato's body must be kept hidden from the world. If anybody knew that Fortunato had injured Montresor a thousand times, then Montresor would be a prime suspect. And if he were a prime suspect, a careful search of his premises would lead to the discovery of Fortunato's body. That is why the reader comes up against a stone wall. Poe intended it that way. He wanted his readers to wonder. It was sufficient that the injuries were known to "You, who so well know the nature of my soul." The story is not addressed to you or me, but to this man or woman who is the only person in the world that Montresor trusts. We can only guess at the nature of those injuries—if we think it is important. They have to be injuries that nobody else knew about. Montresor pretends to be oblivious of any injuries. He deliberately refers to Fortunato as his friend and his good friend on every possible occasion. He constantly addresses Fortunato himself as "my friend" and has even conditioned himself to think of him as such, even when he is plotting to kill him and is leading him to his death. Fortunato must have gotten the idea that this Montresor is a fool and a toady who will put up with anything. Montresor is acting the part of a toady throughout the tale. He keeps addressing Fortunato as "My friend" and treats him with great courtesy. It is noteworthy that he is even obsequious when he is preparing to wall his good friend up inside the little niche.
“Pass your hand,” I said, “over the wall; you cannot help feeling the nitre. Indeed, it is very damp. Once more let me implore you to return. No? Then I must positively leave you. But I must first render you all the little attentions in my power.”