In Edgar Allen Poe's "The Raven," a melancholy student is visited by a raven who utters the famous, cryptic phrase, "Nevermore." Though the student reasons that the bird probably learned the phrase from a past master, he begins to act very strangely in the presence of the bird. More specifically, he begins a sort of dialogue with the raven, culminating with an inquiry about his former love, Lenore, and whether or not he will be reunited with her in the afterlife. The raven, of course, replies "Nevermore," and the student accordingly spirals into despair.
The strange effect here is that the raven's presence causes the narrator to gradually believe that the bird is a sinister omen or some kind of supernatural emissary bent on heightening his depression. In reality, the bird is probably none of those things. As the narrator himself says, it's likely that the bird learned "Nevermore" from a human and dumbly repeats the phrase without a sinister motive (or any motive at all). Though the narrator knows this fact, he continues to assume that the raven is an evil entity trying to torment him. Based on this strange, totally illogical effect, we gain an insight into the depths of the narrator's depression, and we see how, driven by despair and longing for a lost loved one, the human soul insists on needlessly torturing itself.