In Julian Barnes's The Sense of an Ending, Tony Webster admits that he may not be a reliable narrator. He acknowledges that it's probably impossible to tell, objectively, the story of your own life, and that it's therefore up to the reader to question or validate his authority.
The idea of the unreliable narrator has long been an issue in fiction, dating back to medieval times. The term, as a formal literary device, comes from critic Wayne C. Booth's The Rhetoric of Fiction (1961).
There are many reasons why a narrator might be deemed unreliable. The most obvious one is insanity, as in Ken Kesey's One Flew Over the Cuckoo's Nest, or Stephan Benatar's Wish Her Safe At Home. In the case of the latter, the narrator's illness inclines along with the narrative: as the novel and Rachel's mental illness progress, everything is called into question.
Another reason is that the narrator might be a child - too innocent or naïve to provide a reliable perspective for ...