Continuity mistake: The sun as shown in the introduction is visible and rather bright slightly above the horizon. As the shot moves through the clouds (takes only a second or two) and focuses on the houses, the sun is now shielded by clouds and is moved down quite a bit. (00:00:35)
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: When Harry enters the kitchen, the cake/pudding Petunia is fussing over is lopsided and shorter than the one shown in the close-up, when Dudley tries to pick at it. There are subtle differences in the look of the cake/pudding in subsequent shots of it too. (00:01:30)
Continuity mistake: The pillow and blankets on Harry's bed not only change position in consecutive shots, the actual pillow changes drastically from the time Dobby jumps up and down on the bed, until Harry confronts him about the letters. (00:03:05 - 00:04:30)
Continuity mistake: The cork board, hanging over Harry's bed in the Dursley house, significantly changes its position on the wall, as seen when Harry sits on his bed to speak with Dobby. The location of the thumbtack pushed into the top point of the Gryffindor pennant, as well as the angle of the other point, also change throughout the scenes in Harry's bedroom. (00:03:10 - 00:04:45)
Continuity mistake: When Harry first encounters Dobby, as he walks toward the corner of the bed to bow to Harry, a single sock and other clothes are lying on the bed. In the next shot facing Harry, the sock and clothes on the bed have noticeably changed positions and the two long ends of the Gryffindor scarf that hang over the trunk is visible. When Dobby sobs, as he walks off the bed, only one short end of the Gryffindor scarf hangs over the trunk, and it also changes position in following shots. (00:03:10)
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 Dobby comes out of Harry's closet, he has a CG sock on his ear which he throws towards the dresser. The CG sock is not on the floor in the following two shots, although it should be, when the dresser and floor area around it is shown as Dobby says, "Harry Potter musn't be angry with Dobby," as well as when Dobby yells, "No!" and runs out. (00:06:10)
Continuity mistake: In the shot where Dobby takes Harry's letters from his robe and tells him about why he hid them he is standing in front of the dresser, you can see that the drawers in the dresser are neatly closed (with the exception of the bottom one, which is only slightly open). However, in the next shot when Dobby runs out the door with Harry chasing after him you see the dresser again, and this time all the drawers are slightly open and uneven. (00:06:33)
Continuity mistake: When Harry chases Dobby down the stairs at the Dursleys, they stop in the hallway and there is a telephone on the table beside Harry. In the next close-up, the phone and cord's position differ and when Harry rushes past Dobby, the position differs again. (00:06:50)
Continuity mistake: As Harry chases Dobby into the hallway, he lands behind Dobby with knees bent, his left hand and foot in front and his right hand and foot in back. The next shot he has not moved, but his stance is the opposite, left hand and foot in back and his right hand and foot in front. (00:06:55)
Continuity mistake: While standing in the Dursley's kitchen doorway, Dobby says, "Then Dobby must do it sir, for Harry Potter's own good." In the next shot as the cake/pudding floats away, the wine bottle on the counter is pointing towards the hallway. Yet when Harry stands by the kitchen watching the cake/pudding float towards Mrs. Mason, the wine caddy is turned and the bottle now points towards the kitchen wall. (00:07:15)
Continuity mistake: After Dobby drops the cake on the woman's head, Harry's Uncle Vernon says something like "I'm sorry you'll have to forgive my nephew, he's very disturbed," and if you look at his face he's furious. In the next shot, if you look behind Harry you'll see Uncle Vernon acting very casually, and it looks like it's the same scene as when he was telling the joke to his guests. (00:07:50)
Continuity mistake: At the Dursley residence, when Harry wakes up and gets out of bed to look out the window, a bowl and plate with leftovers sit on the desk. In the following shots, until Vernon goes after Harry out the window, the food on the plate changes and the plate changes position back and forth on the table. When Vernon yells, "Petunia, he's escaping," in the interior shot the lamp is at the corner of the bed, yet in the next exterior shot it's lying on the pillow. The position of Harry's comforter changes during his escape as well. (00:08:25)
Continuity mistake: After Harry packs his trunk, in the wide shot, as Ron starts to lean over to attach the hook to the bars, the excess rope is in the corner of Ron's window, but in the next close-up it's right in the center. (00:09:10)
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: 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)
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: Right at the very start when Vernon grabs Harry's ankle during his escape, his grip on the ankle changes instantly. (00:09:25)
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)
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.