Continuity mistake: In the Whomping Willow, the car door on Harry's side recieves a large ripped gash through the metal, along with other assorted gashes alongside it, as is visible when it opens to throw Harry out. Later, when Ron and Harry escape from the spiders, the metal's ripped gash and other dents/gashes (although the initial damage can get worse in the Dark Forest, it cannot change positions) on Harry's door or elsewhere are not only very different than the earlier shots, they are also different within the consecutive shots of this scene. (00:28:00 - 01:48:05)
Visible crew/equipment: Before the car spits out Harry and Ron, when Ron's car door opens as the seat slides out, the black steel bar that his feet rest on is visible under his right foot. Then, when Harry's seat slides out to throw Hedwig out, as the cage flies out the same foot bars are seen by Ron's seat, near the door, in the background. The bars are gone when the seat slides out to toss Scabbers. (00:28:05)
Continuity mistake: In the scene where the car drives away, there are no tracks left in the grass from when the car drove away from the Whomping Willow tree, but the car leaves tire tracks in the grass as it drives away after dumping Ron and Harry. (00:28:05)
Continuity mistake: When Harry and Ron are ejected from the car, the trunk begins to open in this wide shot. Then there are individual close-up shots of Ron and Harry, on the ground. Next is a close-up of the trunk, which is closed and then it opens again. (00:28:05)
Revealing mistake: Harry and Ron's trunks are being thrown out of the car and in the shot just before Harry's close-up, Ron's trunk (the other trunk says HP) slams down on the grass. The top of this trunk has a large perfectly symmetrical hole in the center with four round marks around it, which was probably involved in the throwing somehow. In the wide shot as Hedwig is being tossed out, the trunks are clearly visible and there is no hole in either, nor is there a hole in Ron's trunk at any other time. (00:28:10)
Continuity mistake: When the flying car throws Harry and Ron out of the car, and Harry is lying on the grass, you see that he starts to get up, but in the next wider shot he is starting to get up again. (00:28:10)
Continuity mistake: When the flying car throws Ron and Harry out at Hogwarts, Harry lands on his back. While he lies on his back his sweater has ridden up, exposing his stomach. A few shots later he starts to get up, but a couple of shots after that he's still on his back with his sweater pulled down properly. (00:28:10)
Continuity mistake: When Ron and Harry's belongings are being thrown out of the trunk of the car, the way they fly out and their positions on the ground are different in all of the shots. (00:28:10)
Continuity mistake: Ron and Harry drag their trunks and pets up the steps, to place with the other students' belongings. As Ron places his trunk near the others, the five strips of trim are clearly visible on his trunk, whereas when Ron's trunk is thrown out of the car it has only four strips of trim. Evidently NOT the same trunk. (00:28:10)
Visible crew/equipment: After the Whomping Willow adventure, when the car tosses Scabbers out, in this shot there is a very long METAL BAR attached under the dashboard to the right of the steering column. This metal bar is not in any other shots. (00:28:15)
Continuity mistake: After throwing Harry, Ron and everything else out of the car, Ron's door slams shut. In this close-up the gashes caused by the Whomping Willow are visible along the outside of the door. After the next overhead shot is another close-up of that door as the car speeds away. The gashes seen in this shot are completely different. (00:28:20)
Visible crew/equipment: After ejecting Harry, Ron, Hedwig, Scabbers and all their belongings out of the car, just before Ron's door slams shut, the metal rod attached to the bottom of the door to pull it closed is visible. (00:28:20)
Visible crew/equipment: After the car spits them out and drives itself away and they are chasing after it, if you watch closely you can see the shadow of someone driving the car. (00:28:30)
Continuity mistake: After being tossed out of the car, Ron and Harry chase after it when it drives off. In the wide shots, as they run through the outside corridor and when they stop at the arched doorway, their breath is visible in the air around them. Whereas their breath is not visible in the previous or following shots. (00:28:30)
Continuity mistake: As Ron's owl flies through the hall to deliver the howler during lunchtime, we can see baskets of green apples and pears spaced evenly along the tables. However, after the owl crashes, and for the duration of the scene, the baskets of fruit are missing. (00:28:49)
Continuity mistake: While Ron and Harry are being reprimanded by Snape for the flying car incident, items on Snapes' desk change repeatedly depending on the camera angle. (00:29:35)
Revealing mistake: As Errol swoops down, there are various angles that he's seen flying over students for the drama, however the last ones are very different. First, an overhead wide shot of the dining hall. Second, heading towards the camera, as Fred and George sit on the left. Third, an overhead of the Gryffindor table. The next two differ completely. First a long pan as Errol flies between Gryffindor and Hufflepuff tables, way past Fred, George and Ron's stand-in, who sit in front of the fireplace, causing some students at Gryffindor table to lean FORWARD, so Errol doesn't hit their backs as he goes off screen to the left. Next shot, Errol flies toward the camera passing Fred, George and the rest, who dramatically lean BACKWARD in this shot, as Errol flies over their table between them. (00:33:45)
Continuity mistake: When Errol drops the howler it lands in a bowl of potato chips/crisps. The seal to the envelope is open, but when Ron grabs it the seal is closed. (00:34:00)
Revealing mistake: In the scene where Ron's owl flies over the students, you see that Errol's wings are perfect. However, in the next shot you see all the feathers ruffled (since Errol smashed into so many things). Then they use a dummy owl to ensure safety of the owl. The owl that's lying down is obviously a dummy, too. Then it turns back to the real bird so it can fly away. Mind you, it nearly hits Hermione in the head. (00:34:00)
Continuity mistake: Errol makes a mess with the chips and when Ron deals with the Howler, the mess on the table differs depending on the camera angle. For example, when the Howler says, "Ronald Weasley! How dare...," there is one kind of mess and then when it says, "..and it's entirely your fault!" there is another mess, but when the Howler shreds itself, the mess is the same as the first shot. (00:34:00)
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.