Continuity mistake: On page 186 of the American hardcover, about a quarter of the way down the page, a paragraph begins "Malfoy looked rather as he had done the time Hermione had punched him in the face." However, Hermione never punched Malfoy in the books, she slapped him in Book 3. She only punched him in the third movie.
Continuity mistake: When Severus talks to Narcissa and Bellatrix in his house, he states that he "had sixteen years of information" about Dumbledore when Voldemort returned. However, Voldemort returned in June 1995. Snape would only have been teaching at Hogwarts (and supposedly spying on Dumbledore) for not quite fourteen years at that point, as he started in 1981.