Continuity mistake: In the girls' lavatory, Lockhart tries to bolt so Harry and Ron stop him and push him back. When Harry says, "You first," in this first shot his cloak not only drapes over the sink to his left, it's also under his hand's tight grip of the sink's wall. However in the next shot, still gripping the wall, his cloak now hangs behind him. Then after Ron says, "Better you than us," Lockhart's cloak is positioned the same way as the first shot. (01:59:20)
Continuity mistake: As they fly over Hogwarts Express Ron yells, "Take my hand!" the corner rubber trim of the door frame is peeling up. When Ron grabs hold in the shot facing Ron, their hands are positioned differently than in the next close-up where it's visible that the frame of the door is fairly clean, there are two parcels behind the seat, etc. Yet when Ron grabs hold after Harry says, "...Your hand's all sweaty," the rubber trim is fitted into place, the frame of the door is filthy and there are different parcels behind the seat, etc. (00:25:20)
Continuity mistake: In the duel scene between Harry and Draco, Lockhart walks in front of Harry to cast his spell on the snake. Lockhart is standing between Harry and the snake, though when the snake lands, Lockhart is far behind Harry, back in his original position. (01:08:05)
Continuity mistake: At Harry's bedroom window, the curved dirt line on the windshield is visible when Ron says, "Let's go," in the close-up, as the Ford Anglia begins to turn. In the next two shots as they pull away, the dirt line is different. When Ron yells, "Come on Harry, hurry up," and Harry passes Hedwig to Ron the dirt line is the same as the first shot. This goes back and forth in the remaining shots at the Dursleys. (00:09:15)
Continuity mistake: As Ron and Harry watch in the background, Crabbe and Goyle hold cupcakes that change position, then start eating Hermione's cupcakes. When the close-up faces Crabbe, the little bit of cupcake in Goyle's hand is visible. However, when Goyle rolls his eyes up as he begins to fall backwards, the cupcake in his hand is nearly whole. (01:19:10)
Continuity mistake: The Dursley house is shown in the opening shot, as Harry sits by his window. When Uncle Vernon places bars on Harry's bedroom window, the shrubbery directly below the ground window is completely different. (00:00:50 - 00:07:55)
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: In the scene where the Chamber finally opens, we first see Harry, Ron and Professor Lockhart from the front, and Harry and Ron are slightly behind Lockhart, and in a position to threaten and guard him. When you next see them a moment later from behind, Lockhart is behind them and they are in no position to guard him. (01:58:45)
Continuity mistake: When the car lands in front of the Burrow, it touches the ground just in front of a small shed and passes it within seconds. An angle change then shows the car driving past the same shed. (00:10:40)
Visible crew/equipment: In the Dueling Club scene, as Snape says "Don't move, Potter, I'll get rid of it for you" and walks towards the camera, a film crew chair can be seen on the left side of the screen.
Continuity mistake: There are eleven vertical front bars on Harry's window, five on each side of the center bar. Ron attaches the hook to the bars on Harry's window and when he says "Let's go!" we see the hook between the third and fourth bar on the right. Yet in the next shot and also in the shot looking up as the bars fall, the hook is between the fifth and sixth from the right. (00:09:20)
Continuity mistake: When Harry is grabbed by the Hand of Glory, it grabs by his knuckles. Camera cuts to a view from behind the hand and it has moved a fair way up Harry's hand instantly. (00:15:20)
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.