What are some examples of why you know the narrator is unreliable in the story "The Cask of Amontillado"?
Montresor never mentions even one of the "thousand injuries" that Fortunato has supposedly inflicted upon him:
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.
Montresor also does not detail what the "insult" was. The reader could simply take Montresor's word for it, but that is an assumption. A narrator establishes himself as a reliable source by providing explanations and reasons for his statements. Montresor never does this with the injuries and insults.
Montresor also brags about his ability to hide the truth:
It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will.
Montresor proves how he can lie to and manipulate Fortunato. Since he is directly addressing the reader, who's to say he is not being a manipulative narrator as well?
Some have suggested that Montresor's French name implies that he is new to Italy and therefore, he would have no Italian coat of arms. When Fortunato says he doesn't remember Montresor's coat of arms, this might be a minor insult because Montresor would not have a coat of arms. And note the motto: "Nemo me impune lacessit." This basically means "no one can harm me unpunished." It seems possible that Montresor could have made this up to underscore his feelings of vengeance. We can only guess what else he may have made up.