Continuity mistake: After Lockhart releases the pixies there is an overhead shot of Neville being lifted to the ceiling while students down below start to flee the classroom, including Malfoy and Goyle. In the next shot as Malfoy and Goyle, who are still at the desks, start to run towards the camera, Lockhart is visibly running up the stairs in the background, though in the following shots he's downstairs. (00:36:50)
Continuity mistake: In the scene where Harry and Ron land in the Whomping Willow, the roof of the car gets smashed in. After awhile, the roof is much less damaged. (00:27:30)
Continuity mistake: When Lockhart lets the pixies out, there's a shot with Hermione standing up with a pixie flying at her. When the scene cuts to another shot with a pixie flying in the air, if you look in the background you can see Hermione get up a second time. (00:36:35)
Continuity mistake: In the scene were Mrs. Norris is found petrified, there is a shot of Snape's face and his hair goes down to the middle of his cheek - the rest of his hair is layered like usual. In the next shot of Snape, his hair is longer, almost down to his chin. (00:46:20)
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)
Continuity mistake: When Dobby stands on Harry's desk, in the close-up he slams the lamp base into his head with the electrical cord coming out of the base to his right. Yet in the next close-up the cord comes out of the base directly in front of Dobby and is now wrapped around Dobby's neck. (00:05:25)
Continuity mistake: When the car flies towards the Whomping Willow, Ron holds the wand as both of Harry's hands desperately try to steer the car. Yet in the next shot facing the Whomping Willow, only Ron's hand is visibly on the steering wheel. Then in the next couple of shots Harry is still gripping the steering wheel trying to avoid the tree. (00:26:30)
Continuity mistake: When Harry stabs the basilisk through the roof of the mouth, he pulls his arm out and the fang is not in his arm. Yet a few frames later it appears.
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: 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)
Continuity mistake: After transforming the bird into a water goblet, Professor McGonagall says, "Now it's your turn. Who would like to go first?" The Slytherin girl on the left has pigtails that start at her earlobes, though in the next shot the pigtails start much higher, at the top of her ears. (00:48:35)
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: While in the Whomping Willow, the center point of the cracks and spider cracks around it on the car's windshield changes shape and position. In the first shot of the windshield being smashed to four shots later of Harry turning towards the back, with the windshield visible behind him. Regardless of whether more damage occurs to the windshield, the center point of each of the two cracks should be in the same place, which it is not. (00:27:15)
Continuity mistake: At the Weasleys, there is a close-up of Harry's face as he watches the fry pan being washed. In this shot as Harry walks away, twin 1 removes his jacket, while twin 2, wearing his jacket, closes the window atop the short door. In the next shot as Harry nears the clock, twin 1 takes his jacket off again. (00:11:00)
Continuity mistake: In the hospital wing, as Madame Pomfrey removes the skull bottle top of the Skele-Gro and sets it down, she says, "But it'll be painful," and Harry actually smirks at her. In the next close-up, as she pours some Skele-Gro into a glass and places the bottle down, the skull top now faces the opposite way. In the following shot as she hands Harry the glass, the position of both the skull top and the bottle of Skele-Gro are quite obviously different than the previous close-up. (00:58:55)
Continuity mistake: When Harry stabs the diary with the snake fang, the tip should be full of ink. However, there is no ink on the tip when it is on the floor.
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: While deriding Mr. Weasley in the book shop, Lucius Malfoy snidely holds Ginny's book up and as he says, "But judging by the state of this, I'd say not", in the side shot, he puts the book down and it's loud enough to be heard, yet in the next front shot he still holds it up and brings it down again. This is well before Lucius sneaks Tom's diary into the cauldron behind Ginny's book. (00:21:20)
Continuity mistake: When the Hogwarts letters arrive at the burrow, Percy hands them out. He hands one to Fred and then passes one over Fred to give to George. But in the shot from behind the twins (looking at Harry and Ron), George receives another one. He is at the end of the table, so there is no one for him to pass it to because Arthur obviously didn't get one. (00:13:20)
Continuity mistake: When the bars on Harry's window are seen falling in the shot looking up, the windowsill is up on the left side and is visibly very crooked. Yet when the Weasley car backs up to Harry's window, the windowsill is perfectly fine again. (00:09:20)
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.