About halfway through the book, Don Quixote and Sancho are traveling by donkey, when their animal is stolen. A few pages later, we see the donkey being ridden again! Guess the theif had a moment of conscience, huh? Most likely, this is to be explained in the following way: Cervantes had to write the second part of his book quickly before some other man wrote a second ending which would make fun of the first part. In his rush, Cervantes overlooked that he had already had the donkey stolen, and so, the animal reappears in a fit of forgetfulness.