Visible crew/equipment: When Uncle Vernon falls out of Harry's window, in the interior shot as he yells, "No, no, no, no, no!" leg pads are visible under Vernon's pajamas as his legs fly out the window. (00:10:00)
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)
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)
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: 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: 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)
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: 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: After ejecting Harry, Ron, Hedwig, Scabbers and all their belongings out of the car, just before Ron's door slams shut, the metal rod attached to the bottom of the door to pull it closed is visible. (00:28:20)
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: In the three shots facing Lockhart as he continues to say, "It is my job to arm you against the foulest creatures known to wizardkind... Know only that no harm can befall you whilst I am here," there is a WIRE attached at the bottom left of the cloth that lies on the pixie's cage. However in the close-up of the cage, as Lockhart removes the cloth, the bottom of the cloth is clearly visible and the wire is gone. (00:35:55)
Visible crew/equipment: After Harry, Ron and Hermione discover the bloody writing on the wall and petrified Mrs. Norris other students start arriving. After the overhead shot, at the start of the next close-up as Malfoy, Crabbe and Goyle come round the corner, under Goyle's feet the taped mat is visible on the floor and it was not there in the previous overhead shot. (Visible on fullscreen DVD.) (00:44:35)
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: 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: 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: 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: 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)
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.