Continuity mistake: In several distinctive shots the Whomping Willow severely dents the hood of the car, however the damage to the hood drastically changes in different shots, including during the spider escape. (00:27:10 - 01:48:00)
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: When Ron and Harry crash into the Willow you can see that the Willow smashes the front side of the car and the left light. Afterwards the light is back and undamaged. (00:27:20)
Continuity mistake: In the Whomping Willow, the trunk of the car recieves a noticeable gash just above the lock on the trunk as well as a huge dent in the trunk. However just before the trunk pops open to eject Harry and Ron's belongings, it is blatantly apparent that it is not the same car trunk. The gash and dent are both gone, the Ford Anglia nameplate on the right side is gone, the lock is different, the area under the bent fender is different, etc. After the spider escape, though the nameplate is back, the dent and gash are still gone. (00:27:25)
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: 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)
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)
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)
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)
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)
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)
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)
Continuity mistake: When Errol is lying flat on his back on the table, in the close-up of Errol, Ron takes the Howler out of Errol's beak and the ribbon and seal on the Howler face away from Ron. In the next shot though, still in Ron's left hand as he lifts it, the ribbon and seal on the Howler now face Ron. (00:34:00)
Continuity mistake: When Ron opens up the howler his mom sent him, the howler rises up to face Ron and Ron has direct eye contact with it, but if you watch closely you can see that Harry's eye contact is not correct. (00:34:40)
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.