Continuity mistake: In the Quidditch scene, Harry breaks his right arm and cradles it on his chest, but when he is lying on the ground after he falls off his broom, he then leans directly on it when he sits up to get away from the Bludger. In the shot immediately afterward, he has his hand cradled in front of him again. (00:57:40)
Continuity mistake: After Harry has caught the Snitch and is sitting up, Hermione runs over to him and kneels in the sand next to him. Her robes are covering her knees in a long shot, but as it cuts to a close up her knees can be seen, and this continues to switch for the next couple of shots. (00:57:55)
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)
Continuity mistake: In the hospital wing, as Madame Pomfrey removes the skull bottle top of the Skele-Gro and sets it down, she says, "But it'll be painful," and Harry actually smirks at her. In the next close-up, as she pours some Skele-Gro into a glass and places the bottle down, the skull top now faces the opposite way. In the following shot as she hands Harry the glass, the position of both the skull top and the bottle of Skele-Gro are quite obviously different than the previous close-up. (00:58:55)
Continuity mistake: When Dobby visits Harry in the hospital wing, Harry throws his blankets off and gets out of bed. In the following shots the blankets and pillows change according to the camera angle or is different altogether for that shot. (01:00:30)
Visible crew/equipment: When Harry is in the hospital wing, after Dobby's visit a petrified Colin is carried in. Madame Pomfrey's mark - a mat, is at the foot of Colin's bed, that was not there in the previous shots, when Madame Pomfrey was not there. (01:02:15)
Continuity mistake: Hermione brews the Polyjuice potion, while she sits on the floor with her back to the sinks. When Ron says, "Lucius Malfoy must have opened it when he was at school here," Hermione sits close to the stone's edge on her right which is much closer to the pillar. As well as when Ron asks, "Why are we brewing this potion in broad daylight, in the middle of the girls' lavatory?" she sits closer to the pillar, at an angle. These differ from the other shots. (01:03:10)
Continuity mistake: In the dueling scene with Snape and Lockhart, when Snape says expelliarmus then does his wand movement, the camera leaves him after he has stopped moving his wand to look at how it goes after Lockhart, you can see Snape still in the background and he has not finished moving his wand. (01:05:35)
Continuity mistake: In the scene where Lockhart is attempting to teach students how to cast charms for disarmarment, Professor Snape casts his charms against Lockhart. Lockhart goes flying through the air, wand in hand. The movie then cuts to a shot where Lockharts wand goes flying through the air out of his hands, then cuts back to a shot from his left shoulder where he lands wand in hand. Movie then cuts back to his front left, and he gets up without wand in hand. (01:05:42)
Visible crew/equipment: After Harry says, "Rictusempra!" Malfoy is thrown across the room and lands at Snape's feet. Snape grabs Malfoy at the back of his neck and as he pulls him up, Alan Rickman's mark, the green tape, is visible at his feet, as well as Tom Felton's white tape mark at his left foot. Whereas in the earlier shot, when Snape suggests someone from his own house and says, "Malfoy, perhaps," the end of the mat is visibly without any marks. (Not visible in DVD widescreen format) (01:06:25 - 01:07:40)
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)
Continuity mistake: When Harry cast the charm on Malfoy in the duelling club, Malfoy falls on the floor with is hair messed up. When the camera cuts to the next scene as Snape pulls Malfoy up, his hair is neat again. (01:07:40)
Visible crew/equipment: In the dueling scene, when Snape pulls Malfoy back onto his feet, a cameraman is visible kneeling down on the far left of the screen (Widescreen version only). (01:07:46 - 01:20:00)
Continuity mistake: In the duel scene between Harry and Draco, Lockhart walks in front of Harry to cast his spell on the snake. Lockhart is standing between Harry and the snake, though when the snake lands, Lockhart is far behind Harry, back in his original position. (01:08:05)
Continuity mistake: During Lockhart's dueling demonstration, as Harry speaks Parseltongue, the snake nears Justin Finch-Fletchley, in close-ups and wide shots until after Justin asks, "What are you playing at?" students including Susan, Ernie, etc., on Justin's left, alter their positions in consecutive shots. (Visible on DVD fullscreen.) (01:08:25)
Continuity mistake: In Dumbledore's study, after speaking with the Sorting Hat, Harry walks toward Fawkes. As he passes Dumbledore's desk, on the corner of the desk there are some books lying on the side of the crystal ball. Then in the overhead shot as Harry says, "Your bird... He just caught fire," the books lie diagonally at the corner in front of the crystal ball. (01:14:50)
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)
Continuity mistake: As Ron and Harry watch in the background, Crabbe and Goyle hold cupcakes that change position, then start eating Hermione's cupcakes. When the close-up faces Crabbe, the little bit of cupcake in Goyle's hand is visible. However, when Goyle rolls his eyes up as he begins to fall backwards, the cupcake in his hand is nearly whole. (01:19:10)
Continuity mistake: When Hermione pours the Polyjuice potion into the glasses some of the thick liquid drips down to the top and side of the sink. In both the close-up of Harry's bubbling hand and the shot after the transformation of Goyle/Harry leaning his hands on the sink the Polyjuice potion drips are gone and the sink is clean. The towel near the window hangs differently in this shot, as well as the earlier overhead shot of Harry as he stands at the sink. (Visible on DVD fullscreen.) (01:19:35 - 01:20:45)
Continuity mistake: As Ron and Harry stand in the girls' lavatory, Hermione gives them their glasses of Polyjuice potion. In the wide shot, Ron bends his left index finger to slip through the silver handle at the base of the glass and his right robe sleeve is way past his wrist as they proceed to put hair in the glasses. However in the close-up of Ron's hands, his index finger now goes through the handle the opposite way and a good amount of his right sweater sleeve shows under his robe too. Then when Hermione says, "Cheers," she holds the base of glass with the handle near her thumb, but no fingers go through it as she starts to drink the potion. Yet in her close-up she grips the glass completely differently. (01:19:40)
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.