Audio problem: Hermione takes her wand out in the shot facing her, Harry and Ron, and Hermione's mouth begins to shout the "I" in "Immobulus!" though no sound is heard. It is in the next close-up that she dramatically shouts out the entire word, "Immobulus!" (00:37:40)
Continuity mistake: The cork board, hanging over Harry's bed in the Dursley house, significantly changes its position on the wall, as seen when Harry sits on his bed to speak with Dobby. The location of the thumbtack pushed into the top point of the Gryffindor pennant, as well as the angle of the other point, also change throughout the scenes in Harry's bedroom. (00:03:10 - 00:04:45)
Deliberate mistake: When bars are placed on Harry's windows, so he does not escape, it is apparent that his windows have complete leaded panes of glass. Yet when Harry is being rescued, Ron and Harry speak with each other, even in low voices, through his visibly closed windows. (00:08:00 - 00:09:00)
Continuity mistake: In the Whomping Willow, the trunk of the car recieves a noticeable gash just above the lock on the trunk as well as a huge dent in the trunk. However just before the trunk pops open to eject Harry and Ron's belongings, it is blatantly apparent that it is not the same car trunk. The gash and dent are both gone, the Ford Anglia nameplate on the right side is gone, the lock is different, the area under the bent fender is different, etc. After the spider escape, though the nameplate is back, the dent and gash are still gone. (00:27:25)
Audio problem: After Harry and Ron have found the piece of paper in Hermione's hand, they are walking along a corridor as Harry reads it out. They walk towards the camera and the shot changes and Harry says 'spiders flee before it'. The movement of his lips doesn't match what he says. (01:52:00)
Revealing mistake: As Errol swoops down, there are various angles that he's seen flying over students for the drama, however the last ones are very different. First, an overhead wide shot of the dining hall. Second, heading towards the camera, as Fred and George sit on the left. Third, an overhead of the Gryffindor table. The next two differ completely. First a long pan as Errol flies between Gryffindor and Hufflepuff tables, way past Fred, George and Ron's stand-in, who sit in front of the fireplace, causing some students at Gryffindor table to lean FORWARD, so Errol doesn't hit their backs as he goes off screen to the left. Next shot, Errol flies toward the camera passing Fred, George and the rest, who dramatically lean BACKWARD in this shot, as Errol flies over their table between them. (00:33:45)
Continuity mistake: During the scene when Harry and Ron are talking to Malfoy as Crabbe and Goyle, Draco has his hair slicked straight back. A couple of seconds later, Draco's hair is parted a bit, and messed up at the back. Some more seconds later, his hair is slicked and tidy once more. (01:23:55)
Revealing mistake: When Harry sits in the Gryffindor common room with the diary, in the close-up, the name Tom Marvolo Riddle is gold embossed on the back cover. However in the previous shot, when Harry lifts the diary off the floor in the lavatory, both sides are clearly visible and the name is NOT on either side, nor is it seen on either side when Harry first takes it from Ginny's arm and when he closes it to stab the cover with the Basilisk fang. The name is also NOT embossed on the cover when Dumbledore holds it up in his study or in the close-up of the other side when he places it on the desk in front of Harry and Lucius Malfoy. (01:27:10 - 02:15:10)
Continuity mistake: As Harry chases Dobby into the hallway, he lands behind Dobby with knees bent, his left hand and foot in front and his right hand and foot in back. The next shot he has not moved, but his stance is the opposite, left hand and foot in back and his right hand and foot in front. (00:06:55)
Revealing mistake: When Harry is looking at the photo album Hagrid gave him in his bedroom, there is a clear reflection of the photos in the lenses of his glasses. A moment later in that scene, there is no reflection seen in the lenses, because there are no actual lenses in the frames. (00:01:00)
Visible crew/equipment: In the girls' lavatory Ron pushes Lockhart into the opening of the Chamber and in the shot looking up as Lockhart falls, at the upper right corner the side of a man's FACE is visible near the sink and when Ron jumps something else is there. (01:59:35)
Continuity mistake: The sun as shown in the introduction is visible and rather bright slightly above the horizon. As the shot moves through the clouds (takes only a second or two) and focuses on the houses, the sun is now shielded by clouds and is moved down quite a bit. (00:00:35)
Continuity mistake: Dumbledore asks, "Why then did the Sorting Hat place you in Gryffindor?" and in this close-up the Sorting Hat sits on two books. In the next shot, as Harry answers, "Because I asked it to." The hat now sits on three books in this close-up. During this entire scene in Dumbledore's study, things including the Sorting Hat and lamp behind Dumbledore to his right, change position depending on camera angle. (02:18:00 - 02:20:20)
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: 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: When Harry holds the page about the Basilisk that Hermione tore out of a book, in the two close-ups of that page the bottom left side is crookedly ripped into the words. However, in the wide shots as Harry holds it that side of the page is ripped straight down. (01:52:35)
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)
Visible crew/equipment: As Harry runs from the Basilisk after Riddle shouts, "Parseltongue won't save you now, Potter...," there is a close-up of the Basilisk's face. In the next shot, which faces Harry's back, the overhead studio lighting bar with multiple lights is reflected in the puddle in front of Harry. (02:09:55)
Continuity mistake: In the scene where Harry and Ron land in the Whomping Willow, the roof of the car gets smashed in. After awhile, the roof is much less damaged. (00:27:30)
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)
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.