According to a different source, this question refers to the story "The Tell-Tale Heart" by Edgar Allan Poe.
In this story, we encounter an unreliable narrator. At the beginning of the story, we see that the narrator argues that he is not crazy. He appears to be calm and confident, and we have no reason to doubt his statement. Therefore, we are initially inclined to trust him. However, as the story progresses, and particularly as it gets closer to the end, the narrator's behaviour becomes more erratic and confusing. This suggests that the narrator is actually insane and that we should not believe the things he says.
