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)
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)
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)
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)
Visible crew/equipment: After Harry climbs up the side of Salazar Slytherine's stone head in the Chamber of Secrets, the first time the Basilisk slams his face into the stone in the wide shot, the bottom of a crew member's denim pant leg and shoe are bizarrely visible lying on a work surface, beneath the stone head's beard. (Visible on fullscreen DVD.) (02:13:00)
Visible crew/equipment: While Harry is chased by the Bludger, Hermione says, "No! Even with a proper wand you could hit Harry..." Three shots later, as Harry passes the blue Ravenclaw tower, the back of his robe flies up and the metal base of the seat he sits on is perfectly visible. Some shots are filmed vertically, as he sits on the attached seat with the upright broomstick in front of him, but then edited with the background plate to look as if he is lying on the horizontal broomstick. Hence the need for a seat perpendicular to the broomstick. (00:55:00)
Visible crew/equipment: After the car spits them out and drives itself away and they are chasing after it, if you watch closely you can see the shadow of someone driving the car. (00:28:30)
Visible crew/equipment: While chasing the Snitch, Malfoy says, "You'll never catch me, Potter." In the following wide shot from the side, the seat support under Tom Felton's left thigh is visible, as he was filmed sitting upright for this particular shot. In the following front shot of both together, as Harry passes Malfoy, the seats that are attached to the broomsticks, to add a bit of comfort for Dan Radcliffe and Tom Felton, are visible. (00:56:00)
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)
Visible crew/equipment: Before the car spits out Harry and Ron, when Ron's car door opens as the seat slides out, the black steel bar that his feet rest on is visible under his right foot. Then, when Harry's seat slides out to throw Hedwig out, as the cage flies out the same foot bars are seen by Ron's seat, near the door, in the background. The bars are gone when the seat slides out to toss Scabbers. (00:28:05)
Visible crew/equipment: When Ginny becomes conscious after Harry stabs the book with the fang, as the camera pans back, a semi-clear reflection of an overhead stage light is visible in the ink. (02:16:05)
Visible crew/equipment: After the Whomping Willow adventure, when the car tosses Scabbers out, in this shot there is a very long METAL BAR attached under the dashboard to the right of the steering column. This metal bar is not in any other shots. (00:28:15)
Visible crew/equipment: Ron and Harry discuss the Basilisk's victims and, speaking of Mrs. Norris, Harry says, "There was water on the floor that night. She only saw the Basilisk's reflection." Just before Harry reads aloud from the paper again, the crew's gas torch canister with regulator and tip, that sits in a bucket beside another bucket, leans against the wall. (Visible on DVD fullscreen.) (01:53:10)
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)
Visible crew/equipment: Upon entering the Chamber of Secrets, Harry spots Ginny and runs towards her. After the last close-up of Harry's face as he runs, there is shot that faces Ginny, showing Harry's back. At the very start of this shot, on the bottom left corner of the screen two overhead studio lights, other equipment and scaffolding are perfectly visible, reflected in the puddle to Harry's left. Later when he runs out of the pipe, towards Ginny, some light reflections are visible as well. (Visible on DVD fullscreen.) (02:04:40)
Visible crew/equipment: When Harry and Ron are in the car fleeing from the spiders, there is a close-up of them looking towards the back of the car as they drive backwards. Two shots before the close-up of them yelling, "Wooooh!!!" there is a wide shot of the Dark Forest and the car at the top of the screen just as it starts to jump over the tree roots. In this wide shot at the bottom of the screen, the HEADS of TWO crew members are wonderfully visible! (Visible on fullscreen DVD.) (01:48:20)
Visible crew/equipment: Ron and Harry hide under the invisibility cloak in Hagrid's hut. After Lucius Malfoy enters he starts to walk towards the invisible boys and they nervously back up. As they back up, something large comes up from the bottom of the screen in front of the boys then quickly goes back down. The boys do NOT hold anything when they go under the cloak and they do not hold anything when they come out (fully visible on fullscreen DVD). (01:39:45)
Visible crew/equipment: After Lee announces, "Harry Potter has caught the Snitch. Gryffindor wins!" there is a shot of the falling Bludger. In the next two close-ups of Harry as he avoids the Bludger and looks up, multiple head studio lighting, used to mimic daylight on his features, is very clearly reflected in BOTH lenses of his glasses. In the opening shot of the ongoing Quidditch match, the entire pitch is visible and it is void of any lights. (00:57:30)
Visible crew/equipment: In his study, Dumbledore speaks to Lucius Malfoy and holds up the skewered diary for Malfoy to see. When Dumbledore puts the diary down in this shot as Dobby tugs on Harry's sleeve the moving black wire between Dobby and Malfoy on Dobby's left is visible on the bottom of the screen. This wire was attached to Harry's sleeve to make it move and though the wire between Dobby and Harry was edited out, the other end was not. (Visible on fullscreen DVD.) (02:22:50)
Visible crew/equipment: When Harry and Ron, under the invisibility cloak, knock on the door of Hagrid's hut, Hagrid goes to the door. When Hagrid kicks the door open the MARK, a strip of blue tape, is visible on the floor behind him, near the table. (Visible on DVD fullscreen.) (01:37:45)
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.