Continuity mistake: At Borgin & Burkes Harry comes out of his hiding places and for the next shot his glasses are intact, although for the shots before and hereafter they are broke. (00:19:25)
Continuity mistake: When Harry is in the Chamber of Secrets and he is fighting the basilisk, he drops the sword. The sword starts to fall and by the sound of it, it would have already have fallen off before he catches it. (02:13:40)
Continuity mistake: In the scene where the Chamber finally opens, we first see Harry, Ron and Professor Lockhart from the front, and Harry and Ron are slightly behind Lockhart, and in a position to threaten and guard him. When you next see them a moment later from behind, Lockhart is behind them and they are in no position to guard him. (01:58:45)
Continuity mistake: In the breakfast scene at the Weasleys' house, when Mrs Weasley tells her husband about his sons flying to Harry's house to rescue him, he has something on his fork. Then he asks Harry about rubber ducks, and he cuts up something else. But when he finishes, the only thing on his fork is the original item. (00:13:00)
Continuity mistake: When Harry and Ron are talking to Myrtle when she said somebody threw a book at her she turns around and starts crying. If you look when she starts crying her face is all screwed up. When the camera angle changes when she stops turning and walks off, her face is no longer screwed up. (01:27:00)
Continuity mistake: After being tossed out of the car, Ron and Harry chase after it when it drives off. In the wide shots, as they run through the outside corridor and when they stop at the arched doorway, their breath is visible in the air around them. Whereas their breath is not visible in the previous or following shots. (00:28:30)
Continuity mistake: When Harry and Lockhart are in Lockhart's office, Lockhart has just signed a picture. When they show a side view of Lockhart, the picture appears to not have a signature on it. (00:41:40)
Continuity mistake: In the scenes where Harry & Ron are trying to get away from the spiders, the car windshield goes from not being broken to being broken constantly, from the time it comes over the hill to when it drops them off at school.
Continuity mistake: When the Weasleys and Harry get their Hogwarts letters, there is a shot from behind the twins. If you watch Mrs Weasley you will see that Harry gives her his Hogwarts letter. She holds it with one hand. When the shot changes she is holding it with both of her hands. (00:13:55)
Continuity mistake: When the car lands in front of the Burrow, it touches the ground just in front of a small shed and passes it within seconds. An angle change then shows the car driving past the same shed. (00:10:40)
Continuity mistake: When Harry falls out of the fireplace in the store, his hair appears to have dust all over it, but when he is outside, in Knockturn Alley, his hair is clean. (00:15:15 - 00:16:25)
Continuity mistake: In the scene where Harry puts the sword through the head of the snake you can see that only about half a foot of the sword is covered with blood. Then later in Dumbledore's office, when he asks Harry to take a closer look at the sword, you can see that there is blood over almost the entire sword. (02:13:50 - 02:20:50)
Continuity mistake: Right at the very start when Vernon grabs Harry's ankle during his escape, his grip on the ankle changes instantly. (00:09:25)
Continuity mistake: When Harry drinks the Polyjuice potion you can see his hand, but in the next shot his robe sleeve is over his hand.
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)
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: As Ron's owl flies through the hall to deliver the howler during lunchtime, we can see baskets of green apples and pears spaced evenly along the tables. However, after the owl crashes, and for the duration of the scene, the baskets of fruit are missing. (00:28:49)
Continuity mistake: When Harry first dips his quill into the ink bottle, the bottle is a good distance away from the diary. He drips ink on the page and it disappears. As he flips the page in the next wide shot, the ink bottle is much closer to the diary though he hasn't touched it. (01:27:40)
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)
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.