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)
Audio problem: When the boys sneak into the Weasley house, while Harry walks over to the clock, one of the other boys whispers, "Do you think it'd be alright if we had some of these?" meaning the biscuits on the table, to which another replies, "Yeah. Mum will never know." The problem is, none of the boys are mouthing any of these words. (00:11:05)
Audio problem: At the Burrow, as Harry walks away from the clock, he notices the self knitting needles, in the wide shot as Ron passes the clock, he mouths the words, "It's not much," but nothing is heard. Obviously because it is meant to be heard in one of the following close-ups of Harry when Ron says, "It's not much, but it's home." (00:11:20)
Revealing mistake: Mrs. Weasley might as well knit like a muggle because she's not making any progress with magical knitting. In the Weasley's home and we see the magic knitting, the needle shoves into the hole, but no thread loops around and it is not advancing. The amount that has already been knitted is magic indeed since all the motion shown in the film produces no results. (00:11:25)
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)
Audio problem: At the Weasleys, as they read the Hogwarts' letters, Mrs. Weasley picks up Harry's letter and we hear her say, "We'll manage," though visibly she does not say that, she is saying something else. (Visible on fullscreen DVD - viewed on PC.) (00:13:15)
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)
Revealing mistake: When Harry takes floo powder, in the wide shot Mrs. Weasley grips the pot with her right hand outstretched, her thumb at the top and the other fingers below, but in the next close-up of the pot, the pot is held by both of her hands around the pot. In the close-up of Mrs. Weasley holding the pot, the hands are not Julie Walters' (Mrs. Weasley) hands. (00:14:45)
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)
Continuity mistake: In the scene where Harry slides out from the fireplace into this dark shop, he quickly exits through the front door. Just before he walks out, look closely. The sidewalk outside is bright and sunny, not like the dark and dingy surroundings that he walks out to. (00:16:20)
Continuity mistake: In Knockturn Alley, as Harry rushes to the doorway to step outside the Borgin & Burkes, the advertisements/notices lining the wall across the seedy alley are visible. When Harry turns around after saying, "I'm fine, thank you," the same wall is seen again to his left, but the advertisements all over the wall are completely different now. (00:16:20)
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.