Continuity mistake: When Harry is in the Chamber, running from the Basilisk, he trips and his glasses fly off his head, toward the left of the screen. In the wide shot as Harry trips, when his left hand touches the ground, the glasses are visible above his head. As his right hand touches the ground the glasses are again visible above his head. As the rest of his body touches ground, Harry's feet swing up in the air behind him and the glasses are now in the air way in front of him, just before they fly off to the left of the screen. However, in the next close-up the glasses are right in front of him and bounce toward his face. (02:10:00)
Other mistake: From the direction the light comes from the Chamber, Harry would not have been able to see the shadow of Fawkes blinding the basilisk. (02:10:20)
Continuity mistake: In the Chamber, when Harry runs from the Basilisk he ends up at a gated dead end, with steel bars that are embedded in a circular stone wall. When Harry crouches down, as the Basilisk's face nears, the manner in which the bars are embedded in the stone is actually quite different. (02:11:00)
Continuity mistake: The stripes on the knot of Harry's tie change position between certain shots, yet Harry does not redo his tie. For example, the diagonal red stripe is higher when he first speaks with Tom Riddle and lower when he kills Riddle and thanks Fawkes. (02:11:20 - 02:16:15)
Continuity mistake: When the Basilisk emerges from the water, in the first shot Harry looks at the Sorting Hat and Gryffindor's sword appears. As Harry gets the sword, in the second shot the Sorting Hat is not only positioned differently it is also lying on another part of the 'stone' floor. (02:12:30)
Revealing mistake: When Harry is fighting the Basilisk on top of the skull in the Chamber of Secrets, you can see that there is a safety tip on the end of the sword. (02:12:55)
Visible crew/equipment: After Harry climbs up the side of Salazar Slytherine's stone head in the Chamber of Secrets, the first time the Basilisk slams his face into the stone in the wide shot, the bottom of a crew member's denim pant leg and shoe are bizarrely visible lying on a work surface, beneath the stone head's beard. (Visible on fullscreen DVD.) (02:13:00)
Revealing mistake: As Harry climbs the Salazar Slytherine stone head's locks of hair, the Basilisk crashes his face and creates the first immense hole. As Harry tries to climb higher, it pounds into the stone again creating another huge hole, higher up on another lock of hair, to Harry's left as he hangs on. However, when the Basilisk smashes his face into the stone a third time, as Harry nears the top, the second huge hole is GONE, in this shot and in the first wide shot in which Tom Riddle is visible. (02:13:05)
Continuity mistake: When Harry is in the Chamber of Secrets and he is fighting the basilisk, he drops the sword. The sword starts to fall and by the sound of it, it would have already have fallen off before he catches it. (02:13:40)
Continuity mistake: Harry pulls the sword out from the Basilisk's mouth, clutches at his right arm and the sword he holds has blood only at the tip. In the very next shot, as he clutches the tooth in his right arm, sword still in hand, there is now plenty of blood on the blade. (02:13:50)
Revealing mistake: When Harry puts the sword through the head of the basilisk, you can see that the front pair of fangs are to the left of his right arm that the fang goes through. With the way Harry pulls out the sword it is impossible that the fang would go through his right arm. (02:13:50)
Continuity mistake: In the scene where Harry puts the sword through the head of the snake you can see that only about half a foot of the sword is covered with blood. Then later in Dumbledore's office, when he asks Harry to take a closer look at the sword, you can see that there is blood over almost the entire sword. (02:13:50 - 02:20:50)
Continuity mistake: When the Basilisk falls with his face upside down and dies, there is not much blood at his mouth and on his teeth. However, as Harry passes the Basilisk on his way to Ginny, the snake's mouth and teeth are now covered with a substantial amount of blood. (02:14:10)
Continuity mistake: After Riddle yells, "Stop! No!" Harry stabs the diary the 1st time. In the next shot, as Riddle stares at the hole in his chest, he drops Harry's wand and it bounces near the Sorting Hat (fullscreen edition). In Riddle's close-up he is in the same position, he still stares at the gaping hole in his chest and Harry's wand is again in his hand. (02:15:20)
Continuity mistake: In the scene where Harry is stabbing Tom Riddle's diary for the last time, he closes the book and we see a close up of its cover, covered in ink. In the next close-up of the diary its cover is clean, and a small blue dot (where the ink subsequently squirts from) is visible in the centre for Harry to aim the tooth at. (02:15:45)
Visible crew/equipment: When Ginny becomes conscious after Harry stabs the book with the fang, as the camera pans back, a semi-clear reflection of an overhead stage light is visible in the ink. (02:16:05)
Continuity mistake: There is a scene where Harry is talking to Ginny in the Chamber, and he is holding his arm right below the wound. In this shot Harry has blood on his hand. When Ginny notices the wound, Harry moves his hand up to cover it. Now his hand is spotless. In the very next shot, however, Harry has blood on his hand again. (02:16:30)
Other mistake: This is especially apparent when Harry and company are flying out of the Chamber, but can be seen throughout the movie: when the characters are hanging, their feet are still flat and stiff, as if they are still standing, instead of straightened, as if they are hanging in the air. (02:17:45)
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: 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)
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.