Continuity mistake: When Uncle Vernon pulls on Harry's ankle to stop him from escaping, Vernon's sleeve changes from being ridden up, to at his wrist and then back again in consecutive shots. The amount of pajama sleeve showing under his plaid robe sleeve changes too. (00:09:45)
Continuity mistake: When Harry is in the Weasley's car, Ron yells, "Drive!" and four shots later Uncle Vernon shouts, "No, no, no no no!" when he starts to fall out the window. As the car pulls away from the wall, Harry is not wearing his eyeglasses in this wide shot, though it's on in the previous and following shots. (Most viewers agree that in this shot it is Dan Radcliffe's double in the front seat of the car, who has a longer face than Radcliffe's smaller, rounder face, but that is not the issue of this specific mistake.) (00:09:50)
Continuity mistake: When the car arrives at the Weasley's house, two white ducks are at a small pond, one in the water and one on the bank. From the overhead shot as the car lands, the grass directly around the pond is dry grass and the duck on the grass faces the house. In the next shot from the ground, the grass is very green and that duck faces the other way. (00:10:40)
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)
Continuity mistake: When Harry, Ron, Fred and George arrive at the Weasley house, first in the wide shot as one twin enters the doorway, then in the next close-up as the rest enter and third, just before Ron takes a biscuit, the amount and arrangement of the biscuits differs in each shot. (00:10:45)
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)
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: At the Weasleys, Ron takes his jacket off, picks up a biscuit with his right hand, then switches and holds the jacket in his right hand and the biscuit in his left. In the next shot, a close-up of Harry, Ron takes a bite of the biscuit which is in his right hand again and chews a mouthful, while holding the jacket in his left, yet in the next shot his jacket is in his right hand and he isn't chewing at all. He then takes a second mouthful of the biscuit in his right hand, but in the next shot as he looks down at his biscuit, only a small bite has been taken out of it. (00:11:05)
Continuity mistake: When Harry walks over to the clock at the Burrow, in the wide shot, the hands are all pointing in a particular direction, then in the close-up, some of those hands change their position. However in the next wide shot, as Harry walks away, all of the hands are pointing in the same direction as in the previous wide shot. (00:11:05)
Continuity mistake: As the boys are sitting and eating at the Weasley kitchen table and then joined by Mr. Weasley, in every wide shot Ron sits a good distance away from the table, yet in the close-ups, he's tucked right up against the kitchen table. (Visible in DVD fullscreen format) (00:12:10)
Continuity mistake: When Ginny runs downstairs to ask her mum for her jumper, the boys are eating breakfast at the table, Harry and Ron are sitting on one side, and directly opposite them are Fred, George and Percy sitting on the other side. When they are joined by Arthur Weasley, who sits at the head of the table, as he speaks to Harry and the boys we see the empty space, an unused knife and fork on Arthur's left hand side, though the twins were sitting there eating breakfast, as was seen previously. When Errol comes with the post, the twins are seen at the end of the table once again eating their breakfast, opposite Harry and Ron. (NOT visible on the DVD widescreen) (00:12:40)
Continuity mistake: In the breakfast scene at the Weasleys' house, when Mrs Weasley tells her husband about his sons flying to Harry's house to rescue him, he has something on his fork. Then he asks Harry about rubber ducks, and he cuts up something else. But when he finishes, the only thing on his fork is the original item. (00:13:00)
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: At the Weasley table, twin 1 holds his Hogwarts letter and says, "This lot won't come cheap, mum," while twin 2 (who sits near Mr. Weasley) reaches in front of twin 1, to take his own Hogwarts letter that Percy hands him. In the next shot facing Harry and Ron, as twin 1 continues to say, "The spell books alone are very expensive," twin 2's hand reaches over twin 1 to get his Hogwarts letter from Percy, again, albeit a bit differently. (00:13:50)
Continuity mistake: When the Weasleys and Harry get their Hogwarts letters, there is a shot from behind the twins. If you watch Mrs Weasley you will see that Harry gives her his Hogwarts letter. She holds it with one hand. When the shot changes she is holding it with both of her hands. (00:13:55)
Continuity mistake: When Ron is in the Weasley fireplace, in the first wide shot he starts to take floo powder pinched between the tips of his fingers, yet, in the next close-up he takes a huge palmful of floo powder in his fist. Then in the next wide shot, he has it pinched at the tips of his fingers, however, in the next close-up as he says, "Diagon Alley!" he has a fistful of floo powder yet again. (00:14:20)
Continuity mistake: When Harry falls out of the fireplace in the store, his hair appears to have dust all over it, but when he is outside, in Knockturn Alley, his hair is clean. (00:15:15 - 00:16:25)
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)
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)
Continuity mistake: After Harry arrives at Borgin & Burkes in Knockturn Alley, via floo powder, Harry taps on the glass with the shrunken heads inside. There are different heads in the close-up and wide shot. (00:15:45)
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.