Continuity mistake: When Errol is lying flat on his back on the table, in the close-up of Errol, Ron takes the Howler out of Errol's beak and the ribbon and seal on the Howler face away from Ron. In the next shot though, still in Ron's left hand as he lifts it, the ribbon and seal on the Howler now face Ron. (00:34:00)
Continuity mistake: When Ron opens up the howler his mom sent him, the howler rises up to face Ron and Ron has direct eye contact with it, but if you watch closely you can see that Harry's eye contact is not correct. (00:34:40)
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)
Revealing mistake: When Lockhart says, "I must ask you not to scream. It might..," as he says, "Provoke them," the first shot is a close-up of Lockhart as he begins to remove the cloth. Next is a close-up of the cage as Lockhart reveals the pixies. Lockhart's face is clearly visible and it is not Kenneth Branagh who is mouthing the words, "Provoke them," that were already heard in the previous shot. (00:36:10)
Continuity mistake: When Lockhart lets the pixies out, there's a shot with Hermione standing up with a pixie flying at her. When the scene cuts to another shot with a pixie flying in the air, if you look in the background you can see Hermione get up a second time. (00:36:35)
Continuity mistake: When the Cornish pixies are released, in the close-up, Harry knocks his books onto the floor, Hermione knocks down her quill together with the books, then stands and grips the top of her desk, etc. However in the next wide shot, Harry does NOT knock his books to the floor, Hermione's quill remains on the desk though her books are down on the ground already, she grips the side, not the top, of her desk, etc. (00:36:35)
Continuity mistake: When Wood first spots the Slytherin quidditch team on their way to practice, you can see Ron and Hermione sitting on a bench in the background. You see Hermione put the book she is holding down beside her, and in the close-up, she puts it down again. (00:36:45)
Continuity mistake: After Lockhart releases the pixies there is an overhead shot of Neville being lifted to the ceiling while students down below start to flee the classroom, including Malfoy and Goyle. In the next shot as Malfoy and Goyle, who are still at the desks, start to run towards the camera, Lockhart is visibly running up the stairs in the background, though in the following shots he's downstairs. (00:36:50)
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)
Audio problem: Hermione takes her wand out in the shot facing her, Harry and Ron, and Hermione's mouth begins to shout the "I" in "Immobulus!" though no sound is heard. It is in the next close-up that she dramatically shouts out the entire word, "Immobulus!" (00:37:40)
Continuity mistake: In the close-up when Hermione screams, "Immobulus!" at the Cornish pixies, the floor and platform under Lockhart's desk have a mess of books and papers. Next wide shot the mess on the floor and platform are different. In the next close-up of Hermione, the mess is the same as the first close-up. (00:37:40)
Continuity mistake: When Hermione shouts, "Immobulus!" at the Cornish pixies, in the shot facing Neville, Harry, who's sitting on his knees, is right next to Hermione with Ron at his other side also on his knees. In the next wide shot, Harry and Ron, still on their knees, are much further away from Hermione. (00:37:40)
Audio problem: When we see the first defence against the dark arts class with the pixies taking over, when Lockhart goes out and tells Harry, Ron and Hermione to pop them back into the cage, Hermione makes them all freeze in mid-air. Most of the pixies drop bits of paper, but one drops a big book. You can see it go thundering down the the ground, but you can't hear it land. All you can hear is bits of paper flying everywhere. (00:37:45)
Continuity mistake: When Oliver Wood argues with Flint about who has the pitch for Quidditch practice, there is a close-up of Wood and the ties on his robe. When Ron is on the ground spitting up slugs, in the close-up of Colin, Wood's robe is seen clearly and the ties on his robe are actually put through the grommets and tied very differently, as is the case with others. (00:38:30 - 00:39:35)
Continuity mistake: After Ron's failed attempt to make Draco 'eat slugs', he goes sliding across the grass on his back. However, when he gets on all fours, and when he walks away, there are no dirty marks (mud/grass stains) on his robes. (00:39:15)
Revealing mistake: When Ron is on all fours beginning to cough up slugs, Harry says "Let's take him to Hagrid's". Hermione mouths the lines with him. (00:39:43)
Revealing mistake: In Hagrid's hut, while Ron is still slug sick, Hagrid is comforting Hermione over Draco's mudblood comment. His beard is visibly detached below his lower lip and when he speaks his jaw moves while his beard does not. (00:41:15)
Continuity mistake: When Harry and Lockhart are in Lockhart's office, Lockhart has just signed a picture. When they show a side view of Lockhart, the picture appears to not have a signature on it. (00:41:40)
Continuity mistake: When Harry first hears the whispering voice and is walking down the hallway feeling the wall as the voice gets louder. Notice he has a round red mark on the back of his right hand. In the next shot it's gone. (00:43:00)
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)
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.