I found a site (sigh, did not book mark it) where they document errors in the books. Just the thing for obsessive fans to jump on!
In the first book, after Hagrid appears at the cabin on the island, he takes Harry back to shore on the same boat that Uncle Vernon used to get his family an Harry to the island inthe first place. How did they get back?
In COS, Lockheart talks about a spell that makes werewolves return to normal. Yet in POA and OOTP they deny a cure, and talk about a potion that lets them have minor transformations.