Do You Know Who Your Narrator Is?

Recently, while driving, I got to thinking about perspectives within stories.  You won't believe how often I think about writing while driving. Now by 'perspective', I mean in reference to the narrator's voice. As in the perspective of the narrator.  If you are writing a book in third-person your narrator will probably change (unless you're writing third-person limited). I have seen in a few unpublished stories by young writers, where the authors didn't really take into consideration who the narrator was within a scene or chapter and this led to a disjointed story as the narrator randomly changed mid-scene.