Continuity mistake: When Lockhart lets the pixies out, there's a shot with Hermione standing up with a pixie flying at her. When the scene cuts to another shot with a pixie flying in the air, if you look in the background you can see Hermione get up a second time. (00:36:35)
Continuity mistake: When the Cornish pixies are released, in the close-up, Harry knocks his books onto the floor, Hermione knocks down her quill together with the books, then stands and grips the top of her desk, etc. However in the next wide shot, Harry does NOT knock his books to the floor, Hermione's quill remains on the desk though her books are down on the ground already, she grips the side, not the top, of her desk, etc. (00:36:35)
Continuity mistake: When Wood first spots the Slytherin quidditch team on their way to practice, you can see Ron and Hermione sitting on a bench in the background. You see Hermione put the book she is holding down beside her, and in the close-up, she puts it down again. (00:36:45)
Continuity mistake: After Lockhart releases the pixies there is an overhead shot of Neville being lifted to the ceiling while students down below start to flee the classroom, including Malfoy and Goyle. In the next shot as Malfoy and Goyle, who are still at the desks, start to run towards the camera, Lockhart is visibly running up the stairs in the background, though in the following shots he's downstairs. (00:36:50)
Continuity mistake: In the close-up when Hermione screams, "Immobulus!" at the Cornish pixies, the floor and platform under Lockhart's desk have a mess of books and papers. Next wide shot the mess on the floor and platform are different. In the next close-up of Hermione, the mess is the same as the first close-up. (00:37:40)
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: 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: 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: 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: When Harry first hears the whispering voice and is walking down the hallway feeling the wall as the voice gets louder. Notice he has a round red mark on the back of his right hand. In the next shot it's gone. (00:43:00)
Continuity mistake: In the scene were Mrs. Norris is found petrified, there is a shot of Snape's face and his hair goes down to the middle of his cheek - the rest of his hair is layered like usual. In the next shot of Snape, his hair is longer, almost down to his chin. (00:46:20)
Continuity mistake: In the overhead wide shot of the moving stairs, the camera closes in on Hermione, Harry and Ron on the stairs, with the portraits on the wall, to their left. In this mostly CG shot the small hanging name plaques are missing under the portraits and the portraits on Hermione's left hang much higher over the banister than in the next shot, which has the name plaques in place. (00:47:40)
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: After transforming the bird into a water goblet, Professor McGonagall says, "Now it's your turn. Who would like to go first?" The Slytherin girl on the left has pigtails that start at her earlobes, though in the next shot the pigtails start much higher, at the top of her ears. (00:48:35)
Continuity mistake: In Transfiguration class, when Professor McGonagall says, "Yes, Miss Granger," the Ring-tailed Lemurs are playing at the bottom of their cage, but when McGonagall says, "You all know that Hogwarts was founded over a thousand..." the Lemurs are high on their perch, all nestled together. Then as McGonagall says, "Now, three of the founders coexisted quite harmoniously..." they're playing down below again. Yet, when McGonagall says, "...be kept within all-magic families. In other words, pure-bloods," they're back up on the perch, nestled together again. (00:48:50)
Continuity mistake: As Harry and Malfoy race to the Snitch, in the shot facing Colin as they approach him, the platform he stands on, the stairs to his left, etc., are all visible, as is the area below the platform, just under the Hufflepuff tarp which is in front of Colin, to his left. In the next shot as Harry and Malfoy zoom by, the platform and stairs are different and there is now a crate, with a basket and other things on top of it, sitting under the Hufflepuff tarp, etc. (00:56:25)
Continuity mistake: In the Quidditch match that involves the tampered Bludger, just before he grabs the Snitch the Bludger hits his arm shattering the bone and making the arm guard fly off. For some reason the arm guard flew in the opposite direction from which the Bludger flew, which should be impossible. The Bludger came toward the screen, but the guard knocked away from the screen. (00:57:15)
Continuity mistake: As Harry chases the Snitch after his arm is broken, he reaches and catches it in the shot facing him. In the next side shot, as he begins to roll over while still balancing on top his broom, the attached foot rest is visible between his thighs. However, when he rolls under his broom then skids to the ground, it is clear that the foot rest is gone. Whether the foot rest is in the open or closed position, it would still be visible either above or below his leg. (00:57:20)
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 the Quidditch scene, Harry breaks his right arm and cradles it on his chest, but when he is lying on the ground after he falls off his broom, he then leans directly on it when he sits up to get away from the Bludger. In the shot immediately afterward, he has his hand cradled in front of him again. (00:57:40)
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.