Continuity mistake: In the first overhead shot of Dumbledore's study as he presumably starts to discipline Harry and Ron the corner of the rug below lies flat and evenly under Fawkes' perch. However when Dumbledore stands up to hand Ron the letter for Azkaban the rug differs greatly, though it has not been touched. (02:17:55)
Continuity mistake: In the Slytherine common room, Malfoy sits at the edge of the desk, as he speaks with Crabbe/Ron and Goyle/Harry and the items on the desk are visible around him. When Malfoy says, "It's been fifty years since the Chamber was opened..," he now sits at the left edge, the snake lamp stem is turned around, etc. (01:23:50)
Continuity mistake: While Ron and Harry are being reprimanded by Snape for the flying car incident, items on Snapes' desk change repeatedly depending on the camera angle. (00:29:35)
Continuity mistake: When Harry is in the Gryffindor common room with the diary, the candle in the lantern changes height dramatically. It is short in all shots, yet when the diary transports Harry and when it transports him back, the candle is much taller. (01:28:00)
Continuity mistake: When Harry chases Dobby down the stairs at the Dursleys, they stop in the hallway and there is a telephone on the table beside Harry. In the next close-up, the phone and cord's position differ and when Harry rushes past Dobby, the position differs again. (00:06:50)
Continuity mistake: When Harry enters the kitchen, the cake/pudding Petunia is fussing over is lopsided and shorter than the one shown in the close-up, when Dudley tries to pick at it. There are subtle differences in the look of the cake/pudding in subsequent shots of it too. (00:01:30)
Continuity mistake: As Ron and Harry stand in the girls' lavatory, Hermione gives them their glasses of Polyjuice potion. In the wide shot, Ron bends his left index finger to slip through the silver handle at the base of the glass and his right robe sleeve is way past his wrist as they proceed to put hair in the glasses. However in the close-up of Ron's hands, his index finger now goes through the handle the opposite way and a good amount of his right sweater sleeve shows under his robe too. Then when Hermione says, "Cheers," she holds the base of glass with the handle near her thumb, but no fingers go through it as she starts to drink the potion. Yet in her close-up she grips the glass completely differently. (01:19:40)
Continuity mistake: At the Weasleys, Ron takes his jacket off, picks up a biscuit with his right hand, then switches and holds the jacket in his right hand and the biscuit in his left. In the next shot, a close-up of Harry, Ron takes a bite of the biscuit which is in his right hand again and chews a mouthful, while holding the jacket in his left, yet in the next shot his jacket is in his right hand and he isn't chewing at all. He then takes a second mouthful of the biscuit in his right hand, but in the next shot as he looks down at his biscuit, only a small bite has been taken out of it. (00:11:05)
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: 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 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: While in the girls' lavatory after Harry speaks Parseltongue, in the overhead shot as the sink top rises and in the following close-up of Harry, Ron and Lockhart, the boys stand close to the wall behind them. However, in the next overhead shot as the sinks seperate Harry and Ron are directly in front of the sinks. (01:58:40)
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: When Ron opens up the howler his mom sent him, the howler rises up to face Ron and Ron has direct eye contact with it, but if you watch closely you can see that Harry's eye contact is not correct. (00:34:40)
Continuity mistake: At the Dursley residence, when Harry wakes up and gets out of bed to look out the window, a bowl and plate with leftovers sit on the desk. In the following shots, until Vernon goes after Harry out the window, the food on the plate changes and the plate changes position back and forth on the table. When Vernon yells, "Petunia, he's escaping," in the interior shot the lamp is at the corner of the bed, yet in the next exterior shot it's lying on the pillow. The position of Harry's comforter changes during his escape as well. (00:08:25)
Continuity mistake: In several distinctive shots the Whomping Willow severely dents the hood of the car, however the damage to the hood drastically changes in different shots, including during the spider escape. (00:27:10 - 01:48:00)
Continuity mistake: While in the Slytherin common room, as Malfoy says, "But my father did say this," visible behind Crabbe/Ron and Goyle/Harry, there is a Slytherin striped scarf slung over the back of the chair. This scarf changes position when Ron/Crabbe jumps up and Harry/Goyle grabs him and in the very last overhead shot of Crabbe/Ron and Goyle/Harry as they run up the stairs. (01:24:00)
Continuity mistake: In the Slytherin common room, the green pillow at Malfoy's arm while he sits on the couch differs in the very first shot from behind Malfoy and the ones facing him. The wood logs change behind Malfoy and the marks in the dust on the table between two sofas in front of Goyle differs greatly, after Malfoy asks, "How many times do I have to tell you?" (01:22:30)
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 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)
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.