What should you be aware of when you encounter a first person narrator?
This is an excellent question because it draws attentions to both the limitations and strengths of adopting a first person point of view in narration. Certainly we need to be aware of the great strengths of this approach - it creates a real sense of openness and connection with the reader as we literally see everything through one character's eyes. This can explain the popularity of novels such as To Kill a Mockingbird, as we feel a real connection with Scout and sympathy for her as we share her growth from innocence to maturity step by step as she experiences it herself.
However, we must always be aware that this strength is also its greatest weakness. For a first person point of view will always be partial. Because we see everything from the perspective of one person we always have the danger of being told the story by an unreliable narrator - a narrator that either intentionally or not deceives us because of their perception of the world and the action that is happening. Note that this can be used absolutely masterfully by authors such as Edgar Allen Poe and Henry James, whose use of the unreliable narrator is key to so many of their great stories, when we can tell that what actually is happening is not what is happening in reality. Try novels such as The Turn of the Screw or What Masie Knew or short stories such as "The Tell-Tale Heart" and "The Cask of Amontillado" for excellent examples of the unreliable narrator at its best.
So, whilst the use of this point of view undoubtedly has tremendous strengths, we always need to remember the partial nature of this narration and the potential of the author using an unreliable narrator.