In Edgar Allan Poe's short story "The Cask of Amontillado," one piece of evidence that Montresor is an unreliable narrator concerns his very motive for undertaking the tasks in the story. Montresor opens the story by claiming, "The thousand injuries of Fortunato I had borne as I best could," meaning that Fortunato is guilty of injuring him in a thousand different ways, perhaps financially. Montresor further adds the statement, "... but when he ventured upon insult I vowed revenge," meaning that when Fortunato added insult to injury, Montresor decided he must take revenge.
The very fact that Montressor is vengeful proves that he is an unreliable narrator. To be a reliable narrator, one most be objective; one must view the world from an objective standpoint, meaning a standpoint not influenced by one's personal feelings, prejudices, or biases. An unreliable narrator views the world from a subjective standpoint, meaning a standpoint influenced by personal feelings, prejudices, etc. Since we know Montresor is seeking revenge, we know that he is being driven purely be his emotions, making his viewpoint subjective and himself an unreliable narrator.
Further evidence concerns the fact that the story makes it unclear as to whether of not Fortunato is truly a bad guy. In fact, Montresor describes Fortunato as "a man to be respected and even feared," which calls into question the evilness of Fortunato's character, making us wonder if he really is a character who deserves the fate Montresor has inflicted upon him. By the end of the story, the reader is inclined to think, "no, Fortunato does not deserve such a fate," which makes the ending seem extremely tragic.