Visible crew/equipment: When Harry is in the hospital wing, after Dobby's visit a petrified Colin is carried in. Madame Pomfrey's mark - a mat, is at the foot of Colin's bed, that was not there in the previous shots, when Madame Pomfrey was not there. (01:02:15)
Visible crew/equipment: Upon entering the Chamber of Secrets, Harry spots Ginny and runs towards her. After the last close-up of Harry's face as he runs, there is shot that faces Ginny, showing Harry's back. At the very start of this shot, on the bottom left corner of the screen two overhead studio lights, other equipment and scaffolding are perfectly visible, reflected in the puddle to Harry's left. Later when he runs out of the pipe, towards Ginny, some light reflections are visible as well. (Visible on DVD fullscreen.) (02:04:40)
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)
Visible crew/equipment: When Harry and Ron are in the car fleeing from the spiders, there is a close-up of them looking towards the back of the car as they drive backwards. Two shots before the close-up of them yelling, "Wooooh!!!" there is a wide shot of the Dark Forest and the car at the top of the screen just as it starts to jump over the tree roots. In this wide shot at the bottom of the screen, the HEADS of TWO crew members are wonderfully visible! (Visible on fullscreen DVD.) (01:48:20)
Visible crew/equipment: Ron and Harry hide under the invisibility cloak in Hagrid's hut. After Lucius Malfoy enters he starts to walk towards the invisible boys and they nervously back up. As they back up, something large comes up from the bottom of the screen in front of the boys then quickly goes back down. The boys do NOT hold anything when they go under the cloak and they do not hold anything when they come out (fully visible on fullscreen DVD). (01:39:45)
Continuity mistake: While Harry is communicating with Riddle's diary, after the words "let me take you back fifty years ago" appear on the page, the ink pot's lid is open as the diary pages flip and Harry lifts the book, but in the shot from Harry's POV we see the ink pot lid closed, then the lid is open again when Harry's transported.
Continuity mistake: When Ron and Harry crash into the Willow you can see that the Willow smashes the front side of the car and the left light. Afterwards the light is back and undamaged. (00:27:20)
Visible crew/equipment: After Lee announces, "Harry Potter has caught the Snitch. Gryffindor wins!" there is a shot of the falling Bludger. In the next two close-ups of Harry as he avoids the Bludger and looks up, multiple head studio lighting, used to mimic daylight on his features, is very clearly reflected in BOTH lenses of his glasses. In the opening shot of the ongoing Quidditch match, the entire pitch is visible and it is void of any lights. (00:57:30)
Continuity mistake: When Harry is in the Chamber of Secrets and has just killed the basilisk, you can see in three or four shots that Harry is still on top of Salazar Slytherin's head. Now when the basilisk falls down dead, Riddle looks upward in anger at Harry who should still be up there. However, in the shot immediately following, it shows Harry walking right next to the dead basilisk.
Continuity mistake: When Harry is in Borgin & Burkes in Knockturn Alley, he approaches the glass dome with skulls sitting on the shelf and to the right is a large round mirror sitting atop a cabinet. When he walks away after freeing his hand from The Hand of Glory, in this wide shot there is now a very tall, ornate, wrought iron fixture behind him that stands beside the shelf, in front of the round mirror and cabinet, that was not in that previous shot. (00:15:45)
Plot hole: At the beginning of the movie, Hedwig rattles her cage, and Harry says apologetically, "I can't let you out, Hedwig; I'm not allowed to use magic outside of school." But he wouldn't have to use magic to let her out - the key is visible sticking out of the lock on the cage, so all he would have to do is turn the key. (00:01:10)
Continuity mistake: Tom Riddle says, "She won't wake" and walks toward Harry. When Harry says "Tom. Tom Riddle," the shot faces Harry and Ginny, and Harry's wand is lying on the open ground below the sole of Ginny's shoe. When Harry asks "Are you a ghost?" as the camera pans closer to Harry, his wand is conspicuously in a different place now, it is lying partially hidden under Ginny's robe above her right foot, right next to Harry. In the next shot after Riddle says, "A memory. Preserved...," the wand is gone from the previous shot's position. (Visible on DVD fullscreen.) (02:04:50)
Revealing mistake: Throughout the Quidditch match, Harry's right arm guard is visible. At the top, over his elbow, there is an attached strip of leather that is tied securely around his arm and the three short straps are snugly buckled around his forearm. However, as the Bludger approaches just before breaking his arm, the ties on the strip of leather are GONE, though visibly tucked up in the close-up and the buckled straps are considerably LONGER, deliberately causing the arm guard to be very loose. The position of the arm guard differs in the two close-ups as well. (00:57:10)
Visible crew/equipment: In his study, Dumbledore speaks to Lucius Malfoy and holds up the skewered diary for Malfoy to see. When Dumbledore puts the diary down in this shot as Dobby tugs on Harry's sleeve the moving black wire between Dobby and Malfoy on Dobby's left is visible on the bottom of the screen. This wire was attached to Harry's sleeve to make it move and though the wire between Dobby and Harry was edited out, the other end was not. (Visible on fullscreen DVD.) (02:22:50)
Continuity mistake: When Harry is cornered by the Basilisk the Basilisk is a foot away from his face, but when he reaches to his side to pick up the rock, the Basilisk is a metre (about a yard) away. (02:19:10)
Continuity mistake: When Harry escapes into the Ford Anglia, in the interior shot Ron grabs firm hold of Harry under his left arm, while Vernon pulls on Harry's foot. In the overhead exterior shot Ron's arm holds Harry above his arm, yet in the next close-up of Harry, Ron's arm is under Harry's. Then just after Ron yells, "Drive!" Ron's arm is over Harry's arm once again. (00:09:45)
Visible crew/equipment: When Harry and Ron, under the invisibility cloak, knock on the door of Hagrid's hut, Hagrid goes to the door. When Hagrid kicks the door open the MARK, a strip of blue tape, is visible on the floor behind him, near the table. (Visible on DVD fullscreen.) (01:37:45)
Continuity mistake: When Harry arrives at the Weasley's house, we see a shot of the chimney that he is going to travel up. There is no pot of floo powder nor is there a bracket to hold it. When they go to travel to Diagon Alley, the bracket is on the right of the fireplace attached to the bricks, with the pot of floo powder in it. (00:10:55)
Visible crew/equipment: After Harry, Ron and Hermione discover the bloody writing on the wall and petrified Mrs. Norris other students start arriving. After the overhead shot, at the start of the next close-up as Malfoy, Crabbe and Goyle come round the corner, under Goyle's feet the taped mat is visible on the floor and it was not there in the previous overhead shot. (Visible on fullscreen DVD.) (00:44:35)
Visible crew/equipment: When Harry and Ron are under the invisibility cloak in Hagrid's hut, Hagrid, Dumbledore and Fudge are joined by Lucius Malfoy. In the wide shot as Malfoy steps in, all the mats - their marks - are visible under Malfoy, Fudge and Dumbledore. Then later Dumbledore says, "If the governors desire my removal, I will of course.." and in the shot facing Malfoy as he finishes, "..step aside," the mat that Malfoy now stands on is visible as is the blue mark behind his feet. It's also visible in an earlier shot after he announces the order of suspension. The mat at the foot of Hagrid's chair disappears as well. (Visible on fullscreen DVD.) (01:39:35 - 01:40:55)
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.