Plot hole: Ron and Harry both fly the Ford Car straight into Hogwarts Grounds. However, Dumbledore has strict spells bound onto the Castle border that prevent anything from flying over the castle walls.

Continuity: Dumbledore tells Harry at the end that Lord Voldemort is the last remaining ancestor of Salazar Slytherin. But ancestors are in the past - he means Voldemort is the last descendant of Salazar Slytherin. Not a mistake Dumbledore would have made. (Corrected in later printings.)

Continuity: Hermione says Moaning Myrtle haunts the bathroom on the first floor. However, when Harry sees the writing on the wall outside her bathroom, he is on the second floor.