Visible crew/equipment: After leaving Borgin & Burkes in Knockturn Alley, Harry is accosted by a witch. When he begins to turn around, just as the bald man approaches, the actors' mark is clearly visible on the edge of the first deep step below, with the shallow steps in the background. In the shot from above, after Hagrid says, "What do you think you're doing down here? Come on," their mark is GONE. (Visible on DVD fullscreen.) (00:16:35)
Visible crew/equipment: When Hagrid and Harry leave Knockturn Alley, Hagrid's face is hidden in shadow, as it is really the body double behind Harry and we hear Hagrid's voice say, "You're a mess Harry. Skulking around Knockturn Alley? Dodgy place..." As Hagrid's (Robbie Coltrane) voice continues, "People will think you're up to no good," Dan Radcliffe reaches his white tape mark cueing him to turn and begin to say, "I was lost. I... Hang on. What were you doing down there then?" (Visible on DVD fullscreen.) (00:17:00)
Visible crew/equipment: Just arriving at Diagon Alley Hagrid says, "I was looking for Flesh-Eating Slug Repellent. They're ruining all the school cabbages." In the next shot Hermione exits the shop and runs towards Harry. On the ground is Dan Radcliffe's (Harry) red mark, which he promptly steps on just before Emma Watson (Hermoine) glances down and steps on her own black tape mark. (Visible on fullscreen DVD.) (00:17:20)
Audio problem: While Harry and Hagrid walk down Diagon Alley, as this shot opens Hermione runs toward Harry and just as Harry comes into view he is mouthing his line to Hermione, yet his voice is not heard, only Hermione's voice is heard. (00:17:25)
Continuity mistake: After arriving at Diagon Alley, Hermione runs up to Harry and Hagrid. In the wide shot as she says, "Harry!" there is a man with a red hat, holding an owl, to Harry's left and hanging in the doorway are beads that are pulled to the side door frame. In the close-up when Hermione says, "It's so good to see you," as that man enters the store the beads are hanging straight down. The beads are pulled up again in the next wide shot when they say goodbye to Hagrid. (00:17:25)
Continuity mistake: At Diagon Alley, Hermione says, "Oculus Reparo," and in the next shot facing her, two girls, one with a green coat and one with a blue coat, come up behind her and in the next shot facing Harry as he examines his repaired glasses, they pass behind him. Yet in the next shot facing Hermione, they pass by her again. Then after a shot of Hagrid, in the next shot facing Harry, Hermione and Hagrid, the two girls walk by yet again. (00:17:40)
Continuity mistake: When Harry and Hermione run into Flourish and Blotts, Mrs. Weasley fusses and wipes off Harry's cloak. As she says, "Thank goodness. We'd hoped you'd only gone one grate..," she holds her purse a particular way and Ginny faces Harry, yet, in the next shot as she continues to say, "..too far," she holds her purse differently and Ginny now faces the other way. (00:18:05)
Visible crew/equipment: In Flourish and Blotts, after being spotted, the photographer pushes Harry behind the desk and Lockhart pulls Harry closer. At the start of this shot, the books lying on the floor, on the left, are visible under the desk. There are two strips of white tape, perpendicular to each other, on the floor protruding from under the books. However, in the overhead shot when Lockhart says, "..is currently celebrating its 27th week...," their mark is GONE. The tape is visible again when Lockhart says, "..with my entire collected works...," due to the same camera angle as the first. (Visible in DVD fullscreen) (00:18:35)
Audio problem: At Flourish and Blotts after Lockhart says, "It can't be Harry Potter," the photographer grabs Harry and says, "Excuse me madame," but the photographer's lips do not move. (00:18:35)
Continuity mistake: Harry is forced to stand beside Lockhart as he says, "Nice big smile Harry. Together you and I rate the front page," and the books are visible under the desk. In the overhead shot before it pans to Malfoy, the books are visible again, but the stacked black books' positions differ considerably. (00:18:40)
Visible crew/equipment: In Flourish and Blotts, when Lockhart says "Which, incidentally, is currently celebrating its 27th week atop the Daily Prophet bestseller list," the overhead camera pans up. As the camera passes his feet, Tom Felton's mark (two long strips of white tape) is perfectly visible on the balcony floor where he stands. (00:19:00)
Continuity mistake: At Borgin & Burkes Harry comes out of his hiding places and for the next shot his glasses are intact, although for the shots before and hereafter they are broke. (00:19:25)
Continuity mistake: At Flourish & Blotts, while standing on the steps, Malfoy rips a page out of a book and puts the book down. The position of this book and others, in the piles of books next to it, changes throughout the following shots. (00:19:35)
Continuity mistake: At Flourish and Blotts, when Draco taunts Harry and Ron, he is taller than Harry and his face is quite developed, more so than in the first film. However, in the Duelling Club scene, in the close ups you can see that Harry is the taller one, if only by a little bit, and Draco's face is rounder and more childish. Obviously shot out of sequence. (00:19:55 - 01:06:55)
Continuity mistake: While deriding Mr. Weasley in the book shop, Lucius Malfoy snidely holds Ginny's book up and as he says, "But judging by the state of this, I'd say not", in the side shot, he puts the book down and it's loud enough to be heard, yet in the next front shot he still holds it up and brings it down again. This is well before Lucius sneaks Tom's diary into the cauldron behind Ginny's book. (00:21:20)
Visible crew/equipment: Rushing to make Hogwarts Express at the train station, the shot opens as the camera pans Ginny's trunk and two marks come into view. While Ginny and Mr. Weasley push her trolley, Mr. Weasley is cued and begins to say, "10:58, come on, come on!" just as the actor hits his tape mark on the ground below, while Percy's trolley hits its tape mark, so Harry is visible behind him, and cues Mr. Weasley to continue speaking. In the shot when Mrs. Weasley says, "Okay" for Ginny to go and in the shot as Harry says, "Let's go," their marks are gone. (Visible on DVD fullscreen.) (00:22:15)
Continuity mistake: When Harry and Ron land in the Whomping Willow, one shot from inside the car shows a branch coming through the rear window, and then pulling right out again. In the next shot (from outside the car) the branch is shown still coming out. (00:22:16)
Revealing mistake: When Harry and the Weasleys are standing on Platform 10, it is Ginny's turn to go, in this shot Ron's trunk is clearly visible, note the stains, the latch hardware on the right long side, the corner hardware at the four corners, etc. After Mr. and Mrs. Weasley go, Harry says, "Let's go!" and as they begin to run, Ron's trunk is clearly seen again and it is NOT the same trunk, note the stains, the lack of hardware at the right, no hardware at the four corners, etc, plus the small parcels near Scabbers are different too. (00:22:30)
Revealing mistake: When Harry and Ron run at the wall to get to Platform 9 3/4 if you watch Hedwig's cage when they tumble over you can see a toy bird that is glued onto the perch. (00:22:50)
Continuity mistake: As Ron's luggage cart/trolley slams into Harry's, Ron's trunk is visible. As Ron begins to flip over in the overhead shot, the trunk has five strips of brown trim - two on the edges and three in the middle. When Harry and Ron moan and get up, Ron's trunk now has four strips of brown trim - two on the edges and two in the middle. (00:22:50)
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.