Other mistake: While downstairs Vernon hears Hedwig's squawks, as well as Dobby and Harry's exchange in the bedroom upstairs, distracting enough for them all, including the Masons, to take note. The noise upstairs illicits a reaction from everyone, which Vernon explains is a noisy cat. Yet when Harry very noisily chases Dobby down the steps, Dobby slams into the wall and Harry lands with a loud thud (not remotely 'catlike'), Vernon, who stands on the other side of the shared wall, pleasantly continues his story to his rapt guests, who are also not distracted by the noise. To suggest that such loud noises coming from the hallway right beside them would not draw any reaction from anyone, Vernon and the Masons included, is preposterous.
Other mistake: Dutch DVD Only: After their crashlanding at Hogwarts, Harry and Ron are taken to Snape who tells them the Willow has been standing there "since before they were born". The Dutch translation reads "over a thousand years". It's a minor mistake on itself, but in Book 3 we learn how long the Willow has really been standing there and that's nowhere near "over a thousand years".
Other mistake: This is especially apparent when Harry and company are flying out of the Chamber, but can be seen throughout the movie: when the characters are hanging, their feet are still flat and stiff, as if they are still standing, instead of straightened, as if they are hanging in the air.
Other mistake: Near the end of the film, when Harry is leaning on the sink about to change into Goyle, we see his reflection in the mirror. In the reflection, it shows his face is starting to change and his skin is bubbling up, but his actual face (which we see part of in front of the mirror) is not changing at all.
Other mistake: When Harry is talking to Dobby in his bedroom on Privet Drive, Dobby begins banging his head against the dresser to punish himself. If you look closely, you can see that there is a slight delay between the time that Dobby's computer-animated head appears to hit the dresser, the time you hear the thump, and the time when the drawer moves and the dresser shudders.
Other mistake: Dutch DVD Only: After the duel Harry tells he told the snake to keep away from Justin. In the subtitles, Justin's name appears untranslated. Later when Justin falls prey to the Basilisk he is called by name again but this time his name is translated to Dutch in the subtitles, namely "Joost".
Other mistake: Just before Hermione gives Harry and Ron the two drugged cakes for Crabbe and Goyle in the food hall, snow is falling and to a fairly low level. In spite of this, and it being a continuous shot, as the camera lowers down, the snow ceases, but right at the end of the shot, the camera view raises up a significant amount where we should be able to see the falling snow again, but someone wanted to save pennies on the VFX budget, because the snow has gone.