Continuity mistake: When Harry and Ron run into the wall in the train station, you can see the owl's cage break open and topple sideways, but in the next shots, the cage is closed and standing up. (00:22:50)
Continuity mistake: Harry and Ron crash into the bricks between platforms nine and ten. When Harry crashes, Hedwig's cage slips and the door opens. Then, as Ron's luggage cart hits, Ron falls over and in the close-up as stuffed Hedwig topples over, the entire cage bends and a bar at the top actually breaks. However, in the next close-up of live Hedwig, the bird and cage are fine. (00:22:50)
Continuity mistake: Just as Ron turns downward when Harry says 'we need to find the train', Hedwig looks out the window behind Ron. In the next shot, her entire body is facing the window behind Harry. (00:23:25)
Continuity mistake: When Ron says, "Oh no! The invisibility booster must be faulty," the wipers are shiny chrome, as is the base of the center button between them. When Harry says, "We need to find the train," in the shot of Ron and Harry as the car begins to descend, the shiny chrome of the windshield wipers is GONE. In the Whomping Willow, Ron says, "My wand. Look..." the wipers and the base of the center button are all matte black now. After the tree smashes the windshield, in the next exterior shot, as the camera pans out while the boys scream, the front blue hood of car is seen and the entire wiper is GONE, whereas in the following shot the wiper is back. When the tree comes through the back window, in the front shots, the hood is seen again and the windshield wipers are GONE again. (Visible in DVD fullscreen format.) (00:24:10 - 00:27:00)
Continuity mistake: During the scene where Harry and Ron were looking for the Hogwarts Express, you can see that the bridge is fairly short. During the scene where Harry is clinging to the car, you can see that the train is still on the bridge even though the train would have been across it in about 4-5 seconds. (00:25:15)
Continuity mistake: As they fly over Hogwarts Express Ron yells, "Take my hand!" the corner rubber trim of the door frame is peeling up. When Ron grabs hold in the shot facing Ron, their hands are positioned differently than in the next close-up where it's visible that the frame of the door is fairly clean, there are two parcels behind the seat, etc. Yet when Ron grabs hold after Harry says, "...Your hand's all sweaty," the rubber trim is fitted into place, the frame of the door is filthy and there are different parcels behind the seat, etc. (00:25:20)
Revealing mistake: During the car flight (on the way to Hogwarts) Harry and Ron have gone out of control outside Hogwarts. During the scene when the car is approaching the whomping willow the car is massively out of proportion compared to when the car is in the tree. (00:25:25)
Continuity mistake: While flying over the train, when Harry is climbing back into the flying car, he puts one leg inside. The shot changes and both legs are in the car and he's sitting down. (00:25:35)
Revealing mistake: When Harry and Ron finally get to Hogwarts, they are flying over a lake. The lake reflects Hogwarts, but not the car, which is closer, and directly over it. (00:26:00)
Continuity mistake: When the car flies towards the Whomping Willow, Ron holds the wand as both of Harry's hands desperately try to steer the car. Yet in the next shot facing the Whomping Willow, only Ron's hand is visibly on the steering wheel. Then in the next couple of shots Harry is still gripping the steering wheel trying to avoid the tree. (00:26:30)
Other mistake: When the car crashes in the Whomping Willow, it is turned and moved around a couple of times. Meanwhile, Hedwig is sitting on her perch, not moving at all. (00:26:45)
Visible crew/equipment: While in the Whomping Willow, Ron asks, "What's happening?" and Harry replies, "I don't know." After a few other shots, in the next two front close-ups of the boys in the front seats as they scream, behind Harry we can see a crew member's hand grips the bottom of Hedwig's cage in the back seat, and in the 2nd close-up lays the cage on its side. (Visible on DVD fullscreen.) (00:27:10)
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)
Revealing mistake: While in the Whomping Willow, there is a shot from the interior of the car showing the windshield receiving two large cracks. In the next exterior shot, as the camera pans out while the boys scream, the blue hood of car is seen (with the lack of wiper blades) and the windshield has NO damage whatsoever, as well as when the tree limb comes through the back window, in the two front shots the blue hood is seen and the windshield is intact again. (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)
Revealing mistake: When the limb of the Whomping Willow crashes through the back window of the car, the hole changes shape in certain shots. In the first interior shot, as the branch goes through, the hole that is made is completely different than in the next exterior shot, as the tree branch leaves. (no pun intended) (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)
Visible crew/equipment: While in the Whomping Willow, the window on Ron's side of the car shatters. In the next exterior shot, as the tree limb comes smashing down on the roof, a large piece of the stunt equipment is quite visible at the back bumper of the car. (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)
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.