Continuity mistake: In the scene where Harry slides out from the fireplace into this dark shop, he quickly exits through the front door. Just before he walks out, look closely. The sidewalk outside is bright and sunny, not like the dark and dingy surroundings that he walks out to. (00:16:20)
Continuity mistake: In Knockturn Alley, as Harry rushes to the doorway to step outside the Borgin & Burkes, the advertisements/notices lining the wall across the seedy alley are visible. When Harry turns around after saying, "I'm fine, thank you," the same wall is seen again to his left, but the advertisements all over the wall are completely different now. (00:16:20)
Continuity mistake: After arriving at Diagon Alley, Hermione runs up to Harry and Hagrid. In the wide shot as she says, "Harry!" there is a man with a red hat, holding an owl, to Harry's left and hanging in the doorway are beads that are pulled to the side door frame. In the close-up when Hermione says, "It's so good to see you," as that man enters the store the beads are hanging straight down. The beads are pulled up again in the next wide shot when they say goodbye to Hagrid. (00:17:25)
Continuity mistake: At Diagon Alley, Hermione says, "Oculus Reparo," and in the next shot facing her, two girls, one with a green coat and one with a blue coat, come up behind her and in the next shot facing Harry as he examines his repaired glasses, they pass behind him. Yet in the next shot facing Hermione, they pass by her again. Then after a shot of Hagrid, in the next shot facing Harry, Hermione and Hagrid, the two girls walk by yet again. (00:17:40)
Continuity mistake: When Harry and Hermione run into Flourish and Blotts, Mrs. Weasley fusses and wipes off Harry's cloak. As she says, "Thank goodness. We'd hoped you'd only gone one grate..," she holds her purse a particular way and Ginny faces Harry, yet, in the next shot as she continues to say, "..too far," she holds her purse differently and Ginny now faces the other way. (00:18:05)
Continuity mistake: Harry is forced to stand beside Lockhart as he says, "Nice big smile Harry. Together you and I rate the front page," and the books are visible under the desk. In the overhead shot before it pans to Malfoy, the books are visible again, but the stacked black books' positions differ considerably. (00:18:40)
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: At Flourish & Blotts, while standing on the steps, Malfoy rips a page out of a book and puts the book down. The position of this book and others, in the piles of books next to it, changes throughout the following shots. (00:19:35)
Continuity mistake: At Flourish and Blotts, when Draco taunts Harry and Ron, he is taller than Harry and his face is quite developed, more so than in the first film. However, in the Duelling Club scene, in the close ups you can see that Harry is the taller one, if only by a little bit, and Draco's face is rounder and more childish. Obviously shot out of sequence. (00:19:55 - 01:06:55)
Continuity mistake: While deriding Mr. Weasley in the book shop, Lucius Malfoy snidely holds Ginny's book up and as he says, "But judging by the state of this, I'd say not", in the side shot, he puts the book down and it's loud enough to be heard, yet in the next front shot he still holds it up and brings it down again. This is well before Lucius sneaks Tom's diary into the cauldron behind Ginny's book. (00:21:20)
Continuity mistake: When Harry and Ron land in the Whomping Willow, one shot from inside the car shows a branch coming through the rear window, and then pulling right out again. In the next shot (from outside the car) the branch is shown still coming out. (00:22:16)
Continuity mistake: As Ron's luggage cart/trolley slams into Harry's, Ron's trunk is visible. As Ron begins to flip over in the overhead shot, the trunk has five strips of brown trim - two on the edges and three in the middle. When Harry and Ron moan and get up, Ron's trunk now has four strips of brown trim - two on the edges and two in the middle. (00:22:50)
Continuity mistake: When Harry and Ron run into the wall in the train station, you can see the owl's cage break open and topple sideways, but in the next shots, the cage is closed and standing up. (00:22:50)
Continuity mistake: Harry and Ron crash into the bricks between platforms nine and ten. When Harry crashes, Hedwig's cage slips and the door opens. Then, as Ron's luggage cart hits, Ron falls over and in the close-up as stuffed Hedwig topples over, the entire cage bends and a bar at the top actually breaks. However, in the next close-up of live Hedwig, the bird and cage are fine. (00:22:50)
Continuity mistake: Just as Ron turns downward when Harry says 'we need to find the train', Hedwig looks out the window behind Ron. In the next shot, her entire body is facing the window behind Harry. (00:23:25)
Continuity mistake: When Ron says, "Oh no! The invisibility booster must be faulty," the wipers are shiny chrome, as is the base of the center button between them. When Harry says, "We need to find the train," in the shot of Ron and Harry as the car begins to descend, the shiny chrome of the windshield wipers is GONE. In the Whomping Willow, Ron says, "My wand. Look..." the wipers and the base of the center button are all matte black now. After the tree smashes the windshield, in the next exterior shot, as the camera pans out while the boys scream, the front blue hood of car is seen and the entire wiper is GONE, whereas in the following shot the wiper is back. When the tree comes through the back window, in the front shots, the hood is seen again and the windshield wipers are GONE again. (Visible in DVD fullscreen format.) (00:24:10 - 00:27:00)
Continuity mistake: During the scene where Harry and Ron were looking for the Hogwarts Express, you can see that the bridge is fairly short. During the scene where Harry is clinging to the car, you can see that the train is still on the bridge even though the train would have been across it in about 4-5 seconds. (00:25:15)
Continuity mistake: As they fly over Hogwarts Express Ron yells, "Take my hand!" the corner rubber trim of the door frame is peeling up. When Ron grabs hold in the shot facing Ron, their hands are positioned differently than in the next close-up where it's visible that the frame of the door is fairly clean, there are two parcels behind the seat, etc. Yet when Ron grabs hold after Harry says, "...Your hand's all sweaty," the rubber trim is fitted into place, the frame of the door is filthy and there are different parcels behind the seat, etc. (00:25:20)
Continuity mistake: While flying over the train, when Harry is climbing back into the flying car, he puts one leg inside. The shot changes and both legs are in the car and he's sitting down. (00:25:35)
Continuity mistake: When the car flies towards the Whomping Willow, Ron holds the wand as both of Harry's hands desperately try to steer the car. Yet in the next shot facing the Whomping Willow, only Ron's hand is visibly on the steering wheel. Then in the next couple of shots Harry is still gripping the steering wheel trying to avoid the tree. (00:26:30)
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.