Have you ever had this happen to you? You're reading along in a classic novel (one you've read before). You're sitting in a chair in the living room instead of at your desk because this is your idea of a festive and exciting thing to do during spring break. (Talk about the boring lives of academics!)
All of a sudden you drift into sleep, or a sort of semi-consciousness, in which you're dreaming an alternate plot to the novel--same characters and everything--and the plot makes sense. After about 15 minutes you realize that you're sleeping and start reading again. You're confused because the plot you dreamed is still vivid, and you feel as though you've momentarily lost your place.
Not that this has happened to me, of course ;-).