Continuity mistake: When Lucius Malfoy tries to curse Harry, Dobby stands directly in front of Harry and says, "You shall not harm Harry Potter!" In the side shot of Lucius Malfoy being thrown, Harry is behind Dobby to his right and the grass with its shadow is visible. Though, in the next back shot Harry stands behind Dobby to his left and the grass is again visible, but with no shadow. (02:25:30)
Continuity mistake: When Ron is in the Weasley fireplace, in the first wide shot he starts to take floo powder pinched between the tips of his fingers, yet, in the next close-up he takes a huge palmful of floo powder in his fist. Then in the next wide shot, he has it pinched at the tips of his fingers, however, in the next close-up as he says, "Diagon Alley!" he has a fistful of floo powder yet again. (00:14:20)
Continuity mistake: When the car arrives at the Weasley's house, two white ducks are at a small pond, one in the water and one on the bank. From the overhead shot as the car lands, the grass directly around the pond is dry grass and the duck on the grass faces the house. In the next shot from the ground, the grass is very green and that duck faces the other way. (00:10:40)
Continuity mistake: At Harry's bedroom window, the curved dirt line on the windshield is visible when Ron says, "Let's go," in the close-up, as the Ford Anglia begins to turn. In the next two shots as they pull away, the dirt line is different. When Ron yells, "Come on Harry, hurry up," and Harry passes Hedwig to Ron the dirt line is the same as the first shot. This goes back and forth in the remaining shots at the Dursleys. (00:09:15)
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: After throwing Harry, Ron and everything else out of the car, Ron's door slams shut. In this close-up the gashes caused by the Whomping Willow are visible along the outside of the door. After the next overhead shot is another close-up of that door as the car speeds away. The gashes seen in this shot are completely different. (00:28:20)
Continuity mistake: During Lockhart's dueling demonstration, as Harry speaks Parseltongue, the snake nears Justin Finch-Fletchley, in close-ups and wide shots until after Justin asks, "What are you playing at?" students including Susan, Ernie, etc., on Justin's left, alter their positions in consecutive shots. (Visible on DVD fullscreen.) (01:08:25)
Continuity mistake: In the Chamber of Secrets and afterwards, the stain on Harry's collar is constantly changing. Some examples are the close-up as Harry leans over Ginny saying, "Ginny, oh please don't be dead. Wake up!..." and the close-up when Harry says, "Why do you care how I escaped? Voldemort was after your time." Also, in the close-up when Harry is crouching trapped with his back up against the grate, as the blinded Basilisk comes toward him, and then as he stands and looks at the Basilisk slithering away, up the pipe. (02:04:50)
Continuity mistake: As Ron and Harry watch in the background, Crabbe and Goyle hold cupcakes that change position, then start eating Hermione's cupcakes. When the close-up faces Crabbe, the little bit of cupcake in Goyle's hand is visible. However, when Goyle rolls his eyes up as he begins to fall backwards, the cupcake in his hand is nearly whole. (01:19:10)
Continuity mistake: The car races toward Harry and Ron in the Dark Forest and when it stops, the lantern lies on the ground beside the car's doors. However in the two shots, when Fang gets in (the first time) and when Ron gets in, the lantern is further away from the car. (01:47:55)
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: In the Whomping Willow, the car door on Harry's side recieves a large ripped gash through the metal, along with other assorted gashes alongside it, as is visible when it opens to throw Harry out. Later, when Ron and Harry escape from the spiders, the metal's ripped gash and other dents/gashes (although the initial damage can get worse in the Dark Forest, it cannot change positions) on Harry's door or elsewhere are not only very different than the earlier shots, they are also different within the consecutive shots of this scene. (00:28:00 - 01:48:05)
Continuity mistake: In the Slytherine common room, when Harry and Ron are talking to Malfoy as Crabbe and Goyle, it isn't only Malfoy's hair that's changed. His voice is also higher pitched, suggesting this scene was filmed before his voice dropped, and he is also shorter and has a younger, chubbier face than in the scenes beforehand. Many of the scenes throughout the movie were shot out of sequence. (01:22:55)
Continuity mistake: The Dursley house is shown in the opening shot, as Harry sits by his window. When Uncle Vernon places bars on Harry's bedroom window, the shrubbery directly below the ground window is completely different. (00:00:50 - 00:07:55)
Continuity mistake: When Harry and Ron are ejected from the car, the trunk begins to open in this wide shot. Then there are individual close-up shots of Ron and Harry, on the ground. Next is a close-up of the trunk, which is closed and then it opens again. (00:28:05)
Continuity mistake: At the Weasley table, twin 1 holds his Hogwarts letter and says, "This lot won't come cheap, mum," while twin 2 (who sits near Mr. Weasley) reaches in front of twin 1, to take his own Hogwarts letter that Percy hands him. In the next shot facing Harry and Ron, as twin 1 continues to say, "The spell books alone are very expensive," twin 2's hand reaches over twin 1 to get his Hogwarts letter from Percy, again, albeit a bit differently. (00:13:50)
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: After Dobby drops the cake on the woman's head, Harry's Uncle Vernon says something like "I'm sorry you'll have to forgive my nephew, he's very disturbed," and if you look at his face he's furious. In the next shot, if you look behind Harry you'll see Uncle Vernon acting very casually, and it looks like it's the same scene as when he was telling the joke to his guests. (00:07:50)
Continuity mistake: After Ron's failed attempt to make Draco 'eat slugs', he goes sliding across the grass on his back. However, when he gets on all fours, and when he walks away, there are no dirty marks (mud/grass stains) on his robes. (00:39:15)
Continuity mistake: As Aragog approaches Harry and Ron, Harry says, "Were friends of Hagrid's," and Harry carries the lantern palm side up, at his hip. Yet in the next shot, it's palm side down and the lantern handle is at his shoulder. In the next shot, though it's still at his shoulder it's palm side up now. Then in the two shots facing Aragog, the lantern is down low, but in between those two shots, in the shot facing Harry, his elbow is bent, the lantern is higher up and Fang stands further away. (01:45:15)
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.