Audio problem: Hermione takes her wand out in the shot facing her, Harry and Ron, and Hermione's mouth begins to shout the "I" in "Immobulus!" though no sound is heard. It is in the next close-up that she dramatically shouts out the entire word, "Immobulus!" (00:37:40)
Audio problem: After Harry and Ron have found the piece of paper in Hermione's hand, they are walking along a corridor as Harry reads it out. They walk towards the camera and the shot changes and Harry says 'spiders flee before it'. The movement of his lips doesn't match what he says. (01:52:00)
Audio problem: In the Chamber of Secrets, Riddle says, "You'll find I can be very persuasive." In the next shot facing Harry, the side of Riddle's face is visible and he is obviously speaking before we hear these words, "Not that she knew what she was doing." (02:05:55)
Audio problem: When Harry enters the Chamber of Secrets and spots Ginny lying on the ground, he runs towards her. However, the sound of his footsteps don't match his actual steps. (02:04:40)
Audio problem: Riddle tells young Hagrid in the close-up, "They'll have your wand for this Hagrid. You'll be expelled." In the following side shot Riddle's mouth still moves as Hagrid puts his head down and though no words are heard his lips move as if repeating "You'll be expelled." It is not until the next shot of Harry as he yells, "Hagrid!" that the memory just begins to end as Harry moves back to the present. (01:32:50)
Audio problem: At the Burrow, as Harry walks away from the clock, he notices the self knitting needles, in the wide shot as Ron passes the clock, he mouths the words, "It's not much," but nothing is heard. Obviously because it is meant to be heard in one of the following close-ups of Harry when Ron says, "It's not much, but it's home." (00:11:20)
Audio problem: When we see the first defence against the dark arts class with the pixies taking over, when Lockhart goes out and tells Harry, Ron and Hermione to pop them back into the cage, Hermione makes them all freeze in mid-air. Most of the pixies drop bits of paper, but one drops a big book. You can see it go thundering down the the ground, but you can't hear it land. All you can hear is bits of paper flying everywhere. (00:37:45)
Audio problem: At Flourish and Blotts after Lockhart says, "It can't be Harry Potter," the photographer grabs Harry and says, "Excuse me madame," but the photographer's lips do not move. (00:18:35)
Audio problem: While Harry and Hagrid walk down Diagon Alley, as this shot opens Hermione runs toward Harry and just as Harry comes into view he is mouthing his line to Hermione, yet his voice is not heard, only Hermione's voice is heard. (00:17:25)
Audio problem: At the Weasleys, as they read the Hogwarts' letters, Mrs. Weasley picks up Harry's letter and we hear her say, "We'll manage," though visibly she does not say that, she is saying something else. (Visible on fullscreen DVD - viewed on PC.) (00:13:15)
Audio problem: As Harry escapes into the car, Ron leans forward, puts his left arm in front of Harry and although we hear him say, "I've got you, Harry!" Ron's lips remain apart, with teeth are clenched, and his mouth does not move. (00:09:45)
Audio problem: When Harry, Hermione and Ron discuss Tom Riddle and Hagrid's supposed link to the Chamber of Secrets Hagrid overhears something and asks if they were talking about him. Harry, Hermione and Ron all turn round and we hear them say, "No!" at the same time. If you watch carefully, Hermione says, "No!" slightly afterwards, but no sound comes out. The line has been dubbed so that they all say it in unison. (01:30:16)
Audio problem: When Tom Riddle confronts young Hagrid, the first time Riddle says, "Now, stand aside," Hagrid yells, "No!" but Hagrid's lips don't move. (01:32:25)
Audio problem: When the boys sneak into the Weasley house, while Harry walks over to the clock, one of the other boys whispers, "Do you think it'd be alright if we had some of these?" meaning the biscuits on the table, to which another replies, "Yeah. Mum will never know." The problem is, none of the boys are mouthing any of these words. (00:11:05)
Answer: Yes. It's a common occurrence in a lot of movies for actors to forget their original lines so they improvise something to help move the scene along and not break character. Tom did this because he forgot what his original line was.