Other mistake: When Harry is talking with Arthur Weasley at the Leaky Cauldron, the number '301' (colored in white) digitally pops up right underneath the wanted poster. The composite people neglected to remove it. Sirius Black's number is '390' (colored in black) - it's written on the paper that Sirius holds in the poster. (This occurs in the widescreen version).
Other mistake: During the whomping willow scene, Hermione does indeed say Rupert, not Ron. She says it quickly, but I am not the only one who hears her say Rupert. She definitely does say Rupert. I just listened to it 3 times to make sure. [This may differ between versions - people are equally convinced on both sides.]
Other mistake: When Arthur takes Harry aside at the Leaky Cauldron, the moment before Arthur says, "There are some within the Ministry who would strongly discourage me from divulging what I'm about to reveal..." we can see the digital number "300" flash on the stone pillar to the right of the wanted poster. (This occurs only in the fullscreen version, as opposed to the widescreen version where the number "301" flashes underneath the poster, which is noted in another mistake).
Other mistake: On the Dutch edition of the POA DVD, you can choose either English or Dutch audio. During Snape's werewolf lesson, he demands the students to "turn to page 394". On the Dutch audio version Snape demands "page 349" (considering the way in which the Dutch pronounce numbers an understandable mistake, but still a mistake), and it occurs each time Snape demands for this page. However, the Dutch subtitles oddly enough get the correct translation of the page number.
Other mistake: When Harry and Hermione travel back in time with the Time Turner, Hermione states that the time is 7:30, we then see them run to Hagrid's shack. As the camera passes through the clock on the castle, it shows the time is either 2:00 or 12:10. As well as this we are told that Buckbeak is to be executed at sundown, but he is executed in broad daylight, looking more like midday.