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: When Myrtle is flooding the girls' bathroom, the tap that later doesn't work (the entrance to the Chamber) is happily churning out water - it's on the side nearest the door, so we see it clearly both times. (01:26:05)
Continuity mistake: When Harry and Ron are talking to Myrtle when she said somebody threw a book at her she turns around and starts crying. If you look when she starts crying her face is all screwed up. When the camera angle changes when she stops turning and walks off, her face is no longer screwed up. (01:27:00)
Continuity mistake: While Harry looks through the seemingly empty diary, in the first overhead shot, the quill lies semi diagonally on the table at the top right corner of the diary. In the next shot it lies right next to Harry's right hand. (01:27:20)
Continuity mistake: When Harry first dips his quill into the ink bottle, the bottle is a good distance away from the diary. He drips ink on the page and it disappears. As he flips the page in the next wide shot, the ink bottle is much closer to the diary though he hasn't touched it. (01:27:40)
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 it shows Harry writing in the diary, he writes "My name is Harry Potter." When he does this he says what he is writing. It shows what he is writing until he gets to the letters "Po..." Then the shot cuts out to Harry still continuing his sentence. But he only write about 1 or 2 more letters. That means he couldn't have finished his name. (01:28:10)
Continuity mistake: In the Gryffindor common room, as Harry is preoccupied with the diary, there is a lantern and two stacked books right in front of him. In some shots the lantern is closer to him and in other shots the two books are closer. (01:28:25)
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: 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: In Hagrid's home, when Lucius Malfoy holds out the order of suspension for Dumbledore to take, in the shot facing Malfoy the stiff ribbon ends point in his direction, yet, when the next shot faces Dumbledore, the ribbon ends now point towards Dumbledore. (01:40:15)
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: 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: In the scene where Harry and Ron are in the Dark Forest getting away from the spiders, the car comes and Fang gets in twice. First Fang goes in through Harry's door as the camera faces Harry, then Ron gets in the car and then Fang gets in again through Harry's door as Harry casts his spell. (01:48:00)
Continuity mistake: When Ron and Harry are escaping from the spiders in the flying car the passenger's side window that broke earlier hitting the Whomping Willow is not broken, you can see Ron's reflection. A minute later you see that the window is broken again. (01:48:40)
Continuity mistake: When the spider has Ron by the neck, Harry motions for Ron to move his head so he can take aim at the spider and yells, "Arania Exumai!" As the spider is forced away, in the close-up the back of Ron's head is positioned out his window. In the wide shot of the spider as it 'flies' away, Ron is sitting normally in the car. Yet, in the next close-up Ron's head is still positioned out of his window. (01:48:40)
Continuity mistake: When Harry and Ron are reversing out of the forest to escape the pursuing spiders, Ron spins the car 180 degrees. However, when the shot cuts to inside the car both of them are leaning the wrong way. (01:49:20)
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: In the girls' lavatory, when Harry inspects the sink that Moaning Myrtle points out, the pattern of the stains on the porcelain are all visible. However, when the sink descends the stains are actually different. (01:57:55 - 01:58:55)
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)
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.