Other mistake: Near the end of the film, when Harry is leaning on the sink about to change into Goyle, we see his reflection in the mirror. In the reflection, it shows his face is starting to change and his skin is bubbling up, but his actual face (which we see part of in front of the mirror) is not changing at all. (01:20:40)
Revealing mistake: In the duelling scene, when Harry casts the Rictusempra spell, as Malfoy is flying through the air, he is much larger and obviously the stunt double. (01:07:35)
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)
Other mistake: When Harry and Ron find out Lockhart is a fraud, when Lockhart says: "My boy do you use your common sense? My books wouldn't have sold half as well if people didn't think I'd done all those things". Watch Harry, he's mouthing his lines. (01:56:00)
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)
Revealing mistake: The spectator towers nearest the castle cast shadows on the Quidditch pitch throughout the match, albeit differently in some shots. From within the pitch, when facing the castle, the towers that cast shadows are between Gryffyndor, the last tower on the left and Slytherin the last tower on the right. When Malfoy flies up from below the stands and slams to the ground he lands in front of the opposite side, with the black/white faculty towers, between Ravenclaw on one end and Hufflepuff on the other end, which do not cast shadows on the pitch. Yet, as Harry closes in on the Snitch with his broken arm, in the shot facing the Snitch as his left hand reaches out and in the next shot that faces Harry as he seizes the Snitch, the towers between Ravenclaw and Hufflepuff are the ones that cast shadows on the pitch. (00:56:55)
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)
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)
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.