The strange feeling that you have read something before. I experienced a very strong sense of deja lu recently, when I started reading a new book by a favourite author. It seemed very, very strongly familiar, yet I was certain that I had not read this particular book before. As I got further into the book, the familiarity went away.
What was going on? Had I somehow picked up this book before, read the first chapter, and then put it down again?
It took me a while to work out what had happened, but then I realised: a previous book by the same author that I had read must have done that thing where they print the first chapter from a recent novel by the same author at the end. The real giveaway was that this book then had a chapter from a more recent book by the same author, etc., ad infinitum.
When I was a kid, I was always confused by the Hardy boys books. At the end of each book, the last sentence would be something like, "But adventure did seek out the Hardy boys again, when they encountered 'The Secret of Smugglers' Cove'". This always puzzled me. At the end of the first book, there was a pointer to the second book. And at the end of the second book there was a pointer to the third book. And so on. To my tiny child mind, it seemed impossible to publish the first book until the very last book had been written, since the completion of each book was predicated on the completion of the next. Eventually I solved this problem as well: the Hardy boys books were tedious, repetitive, mass-produced and ghostwritten crap, and so I stopped reading them.