Continuity mistake: When Harry is in the Weasley's car, Ron yells, "Drive!" and four shots later Uncle Vernon shouts, "No, no, no no no!" when he starts to fall out the window. As the car pulls away from the wall, Harry is not wearing his eyeglasses in this wide shot, though it's on in the previous and following shots. (Most viewers agree that in this shot it is Dan Radcliffe's double in the front seat of the car, who has a longer face than Radcliffe's smaller, rounder face, but that is not the issue of this specific mistake.) (00:09:50)
Revealing mistake: When Lockhart says, "I must ask you not to scream. It might..," as he says, "Provoke them," the first shot is a close-up of Lockhart as he begins to remove the cloth. Next is a close-up of the cage as Lockhart reveals the pixies. Lockhart's face is clearly visible and it is not Kenneth Branagh who is mouthing the words, "Provoke them," that were already heard in the previous shot. (00:36:10)
Continuity mistake: There are eleven vertical front bars on Harry's window, five on each side of the center bar. Ron attaches the hook to the bars on Harry's window and when he says "Let's go!" we see the hook between the third and fourth bar on the right. Yet in the next shot and also in the shot looking up as the bars fall, the hook is between the fifth and sixth from the right. (00:09:20)
Continuity mistake: Ron and Harry drag their trunks and pets up the steps, to place with the other students' belongings. As Ron places his trunk near the others, the five strips of trim are clearly visible on his trunk, whereas when Ron's trunk is thrown out of the car it has only four strips of trim. Evidently NOT the same trunk. (00:28:10)
Continuity mistake: When Dobby comes out of Harry's closet, he has a CG sock on his ear which he throws towards the dresser. The CG sock is not on the floor in the following two shots, although it should be, when the dresser and floor area around it is shown as Dobby says, "Harry Potter musn't be angry with Dobby," as well as when Dobby yells, "No!" and runs out. (00:06:10)
Continuity mistake: When Gilderoy jumps into the entrance to the Chamber of Secrets you see his cape go flowing up above his head. In a short second shot, taken from above, it's not. (01:59:40)
Continuity mistake: During breakfast at the Weasleys, when Mr. Weasley asks, "What is the function of a rubber duck?" Ron turns his head toward Harry waiting for his response, but in the next shot Ron is looking down at his food, before they all turn their heads to look out the window at Errol.
Plot hole: When Harry and Ron transform into Crabb and Goyle, he doesn't take off his glasses so then Malfoy can comment on them. Goyle doesn't need glasses - Harry's eyesight should've been immediately blurry, and he would have taken them off.
Other mistake: Harry now has Dudley's toy room and Dudley's is next to his, however when there is a shot outside the house there is only the one window on the left side of the house and not much space between it and the left outside wall, where is the window for Dudley's very narrow room? (00:09:00)
Continuity mistake: Tom Riddle's uniform tie is different to the current ties in the first two Harry Potter films. Yet it matches the style of the students ties in the rest of the films. The mistake here is that Tom Riddle (at school) was before Harry's time at Hogwarts, but I guess when the costume department changed the ties, they didn't realise they should give Tom Riddle an older one as per the first two films.
Continuity mistake: When Snape yells Expelliarmus and shoots it at Lockhart, he puts his left hand behind him. But, on the next overhead shot, his left hand is now stretched out to the side of him.
Continuity mistake: When Harry has Tom Riddle's diary and starts writing into it, he writes "My name is Harry Potter". You can see by the way he writes it he doesn't put a full stop (period) at the end of the sentence. But in the next shot of the diary, there suddenly is a full stop there.
Other mistake: When the spider busts the car window and attacks Ron, we hear chirping and wheezing as the beast tries to impale Ron with its fangs. Yet no fangs are visible, nor piercing into Ron, as he hollers at Harry to 'get going'. When he should have instead, been almost rendered unconscious.
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.
Other mistake: When Hermione is running around the Quidditch field to blow up the bludger, she makes it around the massive Quidditch field in seven seconds flat.