Revealing mistake: At the beginning of the scene near the end of the movie with Lucius Malfoy fuming at Dumbledore in his office, Malfoy's hair is fanned back behind his shoulders. The lighting in the room illuminates the back of his neck, where you can see his real, short brown hair. (02:22:05)
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)
Continuity mistake: When Errol drops the howler it lands in a bowl of potato chips/crisps. The seal to the envelope is open, but when Ron grabs it the seal is closed. (00:34:00)
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)
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)
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)
Revealing mistake: Harry and Ron's trunks are being thrown out of the car and in the shot just before Harry's close-up, Ron's trunk (the other trunk says HP) slams down on the grass. The top of this trunk has a large perfectly symmetrical hole in the center with four round marks around it, which was probably involved in the throwing somehow. In the wide shot as Hedwig is being tossed out, the trunks are clearly visible and there is no hole in either, nor is there a hole in Ron's trunk at any other time. (00:28:10)
Continuity mistake: There is a scene where Harry is talking to Ginny in the Chamber, and he is holding his arm right below the wound. In this shot Harry has blood on his hand. When Ginny notices the wound, Harry moves his hand up to cover it. Now his hand is spotless. In the very next shot, however, Harry has blood on his hand again. (02:16:30)
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)
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)
Revealing mistake: During the Quidditch match, just after the close-ups of Snape and Lucius Malfoy, there is a shot of Harry flying towards the camera, coming to stop, with Draco Malfoy flying in beside him and in this shot the goal posts cast a shadow on the sand. Malfoy remarks, "All right there, Scarhead?" and Harry looks down below at the approaching Bludger. Now in this shot, not only do the goal posts have a completely different shadow, there are no hoops at the top of the poles' shadows (while the angle might make them hard to spot, putting everything in a line, the shadows should at least thicken at the top where the hoops are). (00:54:10)
Revealing mistake: In the scene where the students are in Professor Lockhart's Defence Against the Dark Arts lesson you see that when Professor Lockhart lets out the Cornish Pixies, two of the pixies pull Neville Longbottom up by his ears and hang him on a chandelier. Because the pixies were computer-animated, they had to put clips behind Neville Longbottom's ears to make it look like he was being lifted up by the pixies. When he was dropped onto the chandelier by the pixies, you see his ears are still in a stretched position where the clips are. (00:37:00)
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: 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)
Continuity mistake: In the Chamber, when Harry runs from the Basilisk he ends up at a gated dead end, with steel bars that are embedded in a circular stone wall. When Harry crouches down, as the Basilisk's face nears, the manner in which the bars are embedded in the stone is actually quite different. (02:11:00)
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: 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)
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.