Revealing mistake: When Harry is looking at the photo album Hagrid gave him in his bedroom, there is a clear reflection of the photos in the lenses of his glasses. A moment later in that scene, there is no reflection seen in the lenses, because there are no actual lenses in the frames. (00:01:00)
Revealing mistake: From within the Quidditch pitch, when facing one set of three goal posts the last spectator tower to the left is Ravenclaw and Gryffindor tower is last to the right and when facing the other set of goal posts Slytherin tower is last to the left and Hufflepuff tower is last to the right. After the Bludger breaks Harry's arm, in the shot facing the Snitch, as Harry's hand reaches for it, directly in FRONT of Harry are the goal posts between Slytherin and Hufflepuff towers. Then in the next shot facing Harry, as he reaches and catches Snitch, those same goal posts are directly BEHIND Harry. Yet, when he lands in the sand, he is at those very goal posts. (00:57:20)
Revealing mistake: When Lockhart pushes up Harry's sleeve before attempting to mend Harry's broken bone, we see a few freckles on Harry's forearm. When Lockhart got rid of Harry's bones, he also got rid of Harry's freckles. The freckles are back when Harry drinks the Skele-gro at the hospital wing. In other words, the fake arm was missing details visible on Harry's arm. (00:58:12)
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)
Revealing mistake: In the scene when Harry and Ron come back to the bathroom after drinking the polyjuice potion, they find Hermione turned into a cat. Look at her eyes. When she turns around, a digital editing oval appears around her eyes for the rest of that shot.
Revealing mistake: When Crabbe and Goyle are reaching for the cakes that Harry levitated, Ron and Harry are seen in the background, beside the statue, and though they are a bit out of focus you can clearly see that it is Rupert Grint's body double in each of the long shots. The shape of his face differs and his hair is very floppy. (01:19:00)
Revealing mistake: At the Quidditch match, the goal posts are visible at both ends of the pitch in many shots, including the shortest ones. When rushing to Harry, Hermione shouts, "Finite Incantatem!" In the shot of Ron and Hermione's backs as they run towards Harry, the short goal post's base is visible on the left. It is completely different in this close-up than in any of the wide shots. (00:54:40 - 00:57:45)
Revealing mistake: When the bars on Harry's window are being ripped off, most of the window frame comes off as well. In the close-up. the attached hook pulls the bottom of bars and window outward, causing them to fall a certain way. Yet in the next shot looking up, the way in which the bars and window fall is strikingly different, not to mention unrealistic if it was really being yanked out via the hook at the bottom. (00:09:25)
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)
Revealing mistake: At the Quidditch match, in the close-up when the Bludger knocks off Harry's arm guard, his right arm below the elbow is very conspicuously digitally augmented. When Harry falls toward the sand and unlocks his legs from around the broom, his legs below the knees, especially his right leg, are digitally augmented as well. (Second shot is more visible in DVD fullscreen.) (00:57:10)
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)
Revealing mistake: In the scene where Harry Potter is writing in Tom Riddle's diary, we see Harry dipping his feather pen into the ink jar. Notice the shape and size of Harry's hand. When Harry actually starts to write in the diary we see a close up of Harry's hand, but it's not his actual hand doing the writing. The "stunt" hand has longer thinner fingers than Harry Potter's 12 year old hand. (01:28:05)
Revealing mistake: The appearance of Harry's scar differs in some close-ups. For example, at the Quidditch match, in Harry's first close-up his scar has an outline and is thicker than other shots. Other examples are, in Harry's first close-up as he flies behind Malfoy below the pitch stands and also when he reaches for the Snitch after his arm breaks. (00:53:35 - 00:55:50)
Revealing mistake: After Harry yells, "Wood, look out!" Harry ducks to avoid the Bludger and flies away. In the following close-up of Harry, there are two reflections in each of the lenses of his glasses. The reflections were added deliberately for this shot, however, as he sweeps around the pitch, changing direction as he goes, the reflections should reflect the change in scenery around him, but it does NOT. (00:54:35)
Revealing mistake: When Harry puts the sword through the head of the basilisk, you can see that the front pair of fangs are to the left of his right arm that the fang goes through. With the way Harry pulls out the sword it is impossible that the fang would go through his right arm. (02:13:50)
Revealing mistake: In the duelling scene, when Harry casts the Rictusempra spell, as Malfoy is flying through the air, he is much larger and obviously the stunt double. (01:07:35)
Revealing mistake: The spectator towers nearest the castle cast shadows on the Quidditch pitch throughout the match, albeit differently in some shots. From within the pitch, when facing the castle, the towers that cast shadows are between Gryffyndor, the last tower on the left and Slytherin the last tower on the right. When Malfoy flies up from below the stands and slams to the ground he lands in front of the opposite side, with the black/white faculty towers, between Ravenclaw on one end and Hufflepuff on the other end, which do not cast shadows on the pitch. Yet, as Harry closes in on the Snitch with his broken arm, in the shot facing the Snitch as his left hand reaches out and in the next shot that faces Harry as he seizes the Snitch, the towers between Ravenclaw and Hufflepuff are the ones that cast shadows on the pitch. (00:56:55)
Revealing mistake: In Harry's bedroom, when Harry and Dobby grapple with the lamp, just as Harry says, "Give me the lamp!" Dobby's left hand is between both of Harry's hands gripping the lamp stem, but in the reflection we do not see Dobby's hand, or arm, as he moves about on the desk, nor do we see the fingers of his right hand which grips the base. In this shot what we do see in the reflection are Harry, his hands, the lamp base, stem, shade and cord. No Dobby. (00:05:30)
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)
Revealing mistake: When Lockhart says, "I must ask you not to scream. It might..," as he says, "Provoke them," the first shot is a close-up of Lockhart as he begins to remove the cloth. Next is a close-up of the cage as Lockhart reveals the pixies. Lockhart's face is clearly visible and it is not Kenneth Branagh who is mouthing the words, "Provoke them," that were already heard in the previous shot. (00:36:10)
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.