It seems live every famous author has a chest or book shelf somewhere where he puts all his stinkers. To use an analogy, Babe Ruth was famous for his home runs, but he was also known as the king of strikeouts. Have you written a book in the past five years that you think was a stinker and, if so, what do you think happened to it over the course of writing it that made it end up that way?
I'll go first. My first significant learning experience came when I started writing The Long Winter (a modern fantasy YA novel), I made some horrible foundational decisions because I didn't trust my own voice. Instead, I kept trying to do what various story mentors (such as Dramatica) told me to do. The lesson I learned was that, while an author should never stop learning and practicing the skills of the trade (for example, I'm reading a great book right now called The Psychology Workbook for Writers), the author should never put more faith in other people than the author puts in his own voice. I've learned to be honest with myself and consider the criticism. My own voice might tell me that the criticism is valid. I strive to put my highest faith, as a writer, in God, then in my own voice.