Continuity mistake: When Oliver Wood argues with Flint about who has the pitch for Quidditch practice, there is a close-up of Wood and the ties on his robe. When Ron is on the ground spitting up slugs, in the close-up of Colin, Wood's robe is seen clearly and the ties on his robe are actually put through the grommets and tied very differently, as is the case with others. (00:38:30 - 00:39:35)
Continuity mistake: In the scene where Harry and Ron are in the Dark Forest getting away from the spiders, the car comes and Fang gets in twice. First Fang goes in through Harry's door as the camera faces Harry, then Ron gets in the car and then Fang gets in again through Harry's door as Harry casts his spell. (01:48:00)
Continuity mistake: When it shows Harry writing in the diary, he writes "My name is Harry Potter." When he does this he says what he is writing. It shows what he is writing until he gets to the letters "Po..." Then the shot cuts out to Harry still continuing his sentence. But he only write about 1 or 2 more letters. That means he couldn't have finished his name. (01:28:10)
Continuity mistake: At the Weasley kitchen table, Percy hands the Hogwarts letters to each of the twins. A few shots later, you see one of the twins again being handed the same letter, this time from a different angle.
Continuity mistake: When Harry and Ron are reversing out of the forest to escape the pursuing spiders, Ron spins the car 180 degrees. However, when the shot cuts to inside the car both of them are leaning the wrong way. (01:49:20)
Continuity mistake: When the spider has Ron by the neck, Harry motions for Ron to move his head so he can take aim at the spider and yells, "Arania Exumai!" As the spider is forced away, in the close-up the back of Ron's head is positioned out his window. In the wide shot of the spider as it 'flies' away, Ron is sitting normally in the car. Yet, in the next close-up Ron's head is still positioned out of his window. (01:48:40)
Continuity mistake: When Hermione shouts, "Immobulus!" at the Cornish pixies, in the shot facing Neville, Harry, who's sitting on his knees, is right next to Hermione with Ron at his other side also on his knees. In the next wide shot, Harry and Ron, still on their knees, are much further away from Hermione. (00:37:40)
Continuity mistake: The distance between Harry and Ron when they're sliding into the Chamber of Secrets keeps changing. Harry jumps first, and Ron jumps a few seconds later, so there should be a little distance between them. But when they're sliding down, Ron is directly behind Harry. At the bottom though, Harry slides onto the ground a few seconds before Ron, meaning they're again some distance apart. (02:00:10)
Continuity mistake: In the Gryffindor common room, as Harry is preoccupied with the diary, there is a lantern and two stacked books right in front of him. In some shots the lantern is closer to him and in other shots the two books are closer. (01:28:25)
Continuity mistake: In the scene where McGonagall explains about the Chamber of Secrets there is a close up of Harry, and she obviously walks past him as you see her green robes block him from view for a second. However, the next shot shows her from the front walking towards the camera and you can see her walking past Malfoy, Crabbe and Goyle who are sitting behind Harry. (00:48:10)
Continuity mistake: At the Quidditch match, in the overhead close-up of Harry when he lands, the area of sand around him is visible. When Harry lifts up the Snitch and smiles, and in the next overhead shot as he begins to roll out of the way of the Bludger, the sand is still visible around his head. The tracks in the sand are different in each of these shots, but it's not been touched yet. (00:57:25)
Continuity mistake: In alternating shots while Harry is in the Chamber of Secrets, he has a pimple above his lip; in other shots he does not. (02:06:40 - 02:11:30)
Continuity mistake: When Harry first encounters Dobby, as he walks toward the corner of the bed to bow to Harry, a single sock and other clothes are lying on the bed. In the next shot facing Harry, the sock and clothes on the bed have noticeably changed positions and the two long ends of the Gryffindor scarf that hang over the trunk is visible. When Dobby sobs, as he walks off the bed, only one short end of the Gryffindor scarf hangs over the trunk, and it also changes position in following shots. (00:03:10)
Continuity mistake: The smudges on Harry's face and the dirt on Harry's cloak, pants, etc., change in consecutive shots, throughout the shots after the rock slide, within the Chamber of Secrets and thereafter. (02:04:00)
Continuity mistake: While Harry looks through the seemingly empty diary, in the first overhead shot, the quill lies semi diagonally on the table at the top right corner of the diary. In the next shot it lies right next to Harry's right hand. (01:27:20)
Continuity mistake: Errol makes a mess with the chips and when Ron deals with the Howler, the mess on the table differs depending on the camera angle. For example, when the Howler says, "Ronald Weasley! How dare...," there is one kind of mess and then when it says, "..and it's entirely your fault!" there is another mess, but when the Howler shreds itself, the mess is the same as the first shot. (00:34:00)
Continuity mistake: In the duel scene between Harry and Draco, Lockhart walks in front of Harry to cast his spell on the snake. Lockhart is standing between Harry and the snake, though when the snake lands, Lockhart is far behind Harry, back in his original position. (01:08:05)
Continuity mistake: In the second to last scene when Dumbledore is talking to Harry and Ron, Ron's hair is a bit roughed and has a big cowlick. In the next shot, the cowlick is gone and both Ron and Harry's hair is neat. (02:18:20)
Continuity mistake: In the dueling scene with Snape and Lockhart, when Snape says expelliarmus then does his wand movement, the camera leaves him after he has stopped moving his wand to look at how it goes after Lockhart, you can see Snape still in the background and he has not finished moving his wand. (01:05:35)
Continuity mistake: In Dumbledore's study, after speaking with the Sorting Hat, Harry walks toward Fawkes. As he passes Dumbledore's desk, on the corner of the desk there are some books lying on the side of the crystal ball. Then in the overhead shot as Harry says, "Your bird... He just caught fire," the books lie diagonally at the corner in front of the crystal ball. (01:14:50)
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.