Continuity mistake: When Ron and Harry's belongings are being thrown out of the trunk of the car, the way they fly out and their positions on the ground are different in all of the shots. (00:28:10)
Continuity mistake: After arriving at Diagon Alley, Hermione runs up to Harry and Hagrid. In the wide shot as she says, "Harry!" there is a man with a red hat, holding an owl, to Harry's left and hanging in the doorway are beads that are pulled to the side door frame. In the close-up when Hermione says, "It's so good to see you," as that man enters the store the beads are hanging straight down. The beads are pulled up again in the next wide shot when they say goodbye to Hagrid. (00:17:25)
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: Just as Ron turns downward when Harry says 'we need to find the train', Hedwig looks out the window behind Ron. In the next shot, her entire body is facing the window behind Harry. (00:23:25)
Continuity mistake: When Harry stabs the diary, it is completely soaked in ink, but when Professor Dumbeldore shows it to him it is partially dry. I doubt that Harry had time to escape the chamber, try and wash off the ink, and go to Dumbeldore's office. We see that stabbing the diary renders it powerless, so it can't have soaked up all the ink.
Continuity mistake: In the scene where the car drives away, there are no tracks left in the grass from when the car drove away from the Whomping Willow tree, but the car leaves tire tracks in the grass as it drives away after dumping Ron and Harry. (00:28:05)
Continuity mistake: When Tom Riddle takes Harry's wand in the Chamber of Secrets and Harry says "Give me my wand, Tom," his arm goes from being at his side to outstretched to back at his side in successive shots. (02:05:30)
Continuity mistake: When Ginny runs downstairs to ask her mum for her jumper, the boys are eating breakfast at the table, Harry and Ron are sitting on one side, and directly opposite them are Fred, George and Percy sitting on the other side. When they are joined by Arthur Weasley, who sits at the head of the table, as he speaks to Harry and the boys we see the empty space, an unused knife and fork on Arthur's left hand side, though the twins were sitting there eating breakfast, as was seen previously. When Errol comes with the post, the twins are seen at the end of the table once again eating their breakfast, opposite Harry and Ron. (NOT visible on the DVD widescreen) (00:12:40)
Continuity mistake: When Harry just enters the Chamber he is running towards Ginny and drops his wand behind him, then Tom Riddle comes out and is to the front/side of Harry,he then picks up Harry's wand but he never walked behind Harry to pick it up, so the wand was in the wrong place. (02:04:45)
Continuity mistake: Hermione brews the Polyjuice potion, while she sits on the floor with her back to the sinks. When Ron says, "Lucius Malfoy must have opened it when he was at school here," Hermione sits close to the stone's edge on her right which is much closer to the pillar. As well as when Ron asks, "Why are we brewing this potion in broad daylight, in the middle of the girls' lavatory?" she sits closer to the pillar, at an angle. These differ from the other shots. (01:03:10)
Continuity mistake: When we see Harry writing in Tom Riddle's diary, as he writes, the feather end of his quill is pointing to the right. But when we see the three close ups of Harry's hand writing the words, "Do you know anything about the Chamber of Secrets? Can you tell me?" in the diary, the feather end of the quill is pointing towards Harry. (01:28:25)
Continuity mistake: When Errol is lying flat on his back on the table, in the close-up of Errol, Ron takes the Howler out of Errol's beak and the ribbon and seal on the Howler face away from Ron. In the next shot though, still in Ron's left hand as he lifts it, the ribbon and seal on the Howler now face Ron. (00:34:00)
Continuity mistake: When Lucius Malfoy charges into Dumbledore's study, in the overhead shot as he says, "So it's true. You have returned," the diary is visible and it barely has a shadow on the desk, yet in the next shot, a close-up of Dumbledore, the diary casts a long shadow. When Lucius leaves, Harry begins to ask, "Sir...," in the overhead shot, the diary sits at the top of Dumbledore's desk, yet in the next shot, a close-up of the diary, it sits close to the bottom. (02:21:35 - 02:24:00)
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 Harry, Ron, Fred and George arrive at the Weasley house, first in the wide shot as one twin enters the doorway, then in the next close-up as the rest enter and third, just before Ron takes a biscuit, the amount and arrangement of the biscuits differs in each shot. (00:10:45)
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: Throughout the film, one of the young actors may have one or more pimples on their face, yet in the following shots which may take place only moments later, the pimples are gone, or visa versa. For example, when Hagrid joins Harry, Hermione and Ron outside when he overhears, "Mad and hairy...," Harry has one on his cheek. Neville tells them to rush to Harry's room. While Harry, whose own pimple has disappeared, searches the mess, the one on Ron's chin is visible, though it wasn't in the shot outside. Scenes are shot out of sequence, hence the erratic complexion continuity problem. (01:33:50 - 01:35:00)
Continuity mistake: When Harry says, "Tom. Tom Riddle," the shot faces Harry and Ginny, and Ginny's foot is parallel to the ground. In the next shot facing Riddle, Ginny's foot is perpendicular to the ground. When Harry asks, "Are you a ghost?" Ginny's foot is parallel. When Riddle says, "As poor Ginny grows weaker...," her foot is perpendicular to the ground once more and so forth. (02:04:50)
Continuity mistake: In Dumbledore's study, when Harry is handed Godric Gryffindor's sword, in the close-up the name written down the length of the sword is clean, with blood stains only below the name. Yet in the previous and following shots, the blood stains are much closer to the hilt of the sword where the name is. (02:20:50)
Continuity mistake: When Uncle Vernon pulls on Harry's ankle to stop him from escaping, Vernon's sleeve changes from being ridden up, to at his wrist and then back again in consecutive shots. The amount of pajama sleeve showing under his plaid robe sleeve changes too. (00:09:45)
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.