The author may have forgotten the placeholder name she used for some minor character, or the hair color of a child, or insignificant details of the world. Unfortunately the readers will not have forgotten and the author will be in trouble if facts have changed between book one and book three, unless there is an excellent reason which is explained in the book.
I recommend keeping a series "bible". This may be as simple as a spreadsheet with character names and defining characteristics on it, or as complicated as a book with every detail included. That's up to the author.
What is essential, however, is that Tim does not become Tom, a blue flower does not miraculously turn red, and the poisonous snakes that terrorised the villagers in book one do not suddenly disappear from the series without a convincing explanation.