Harry Potter and the Chamber of Secrets

Mistakes:

When Harry breaks his right arm playing Quidditch, he takes great care to keep it held up as he rolls from side to side to avoid the bludger, but at one point he puts his weight on it without any apparent discomfort. In the next shot he is holding it across his stomach again.
When Harry and Hagrid hug (when Hagrid has just returned from Azkaban), the flames in the bowls hanging on the wall behind them burn backward briefly.
the Quidditch match, the wire that attaches the broom to Harry is visible for several seconds.
Ron receives the "Howler" from his mother. After his owl, Errol crashes into the table, you can see that the red envelope is open, as Ron removes it from Errol's mouth. We then see Ron breaking the seal on this envelope so he can read it.
When Harry and Ron's car is caught by the tree, a limb smashes a hole in the rear window but the hole is missing in one shot when the car tips forward while falling out of the tree.
When Harry and Ron are in the Dark Forest and the spiders begin to attack, Fang gets in the car twice.
During the scene when you first meet Moaning Myrtle and she is on top of the girls sink the cauldron is smoking but when she flies down into the toilet it is not smoking.
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.
When Harry and Ron find out Lockhart is a fraud, when Lockhart says, "My boy do you use your common sense? My books wouldn't have sold half as well if people didn't think I'd done all those things." Harry is mouthing his lines.
When Harry is chasing after Dobby in his house and they get to the downstairs hallway they both stop running. Harry stops and he has one leg in front of him. In the next shot, the other leg is in front of him instead.
When the Weasleys' enchanted car has ejected them and Ron and Harry are chasing the car, you can see the hand and silhouette of the driver.
When Mr. Weasley first gets home, Fred and George are sitting in the two seats nearest the door. When he first talks to Harry the seat next to him is empty. When Errol flies in with the mail Fred and George are sitting next to him again.
After retrieving the paper from Hermione, Harry, who seems to be reading off it, says "...spiders flee before it." His mouth doesn't match the soundtrack.
Near the end of the film when Harry and Ron have discovered Lockhart's a fake, they push him towards the washstand in the girls' bathroom, when Lockhart touches this it shakes quite considerably (proving it is not stone).
During the scene with Dobby the Houseelf in Harry's bedroom, there is a single lamp providing light in the room. However when Dobby pushes the stool to climb onto it, the shadow he projects actually leans TOWARDS the light, as opposed to away from it. All the shadows created by the natural light coincide to the light's position, but the CGI shadows Dobby creates don't match up.
When Prof. Lockhart is knocked backwards by Prof. Snape's charm during their duel, his wand can be seen flying through the air. When he lands, however, the wand is back in his hand.
When Harry and Ron are flying the car, after they nearly miss getting hit by the train, Harry falls out of the car and grabs the door handle. Just before we cut back to Ron, we see Harry lose his grip on the handle completely, but the next time we see him, he's hanging on again.
Just after Harry and Ron realize the basilisk is getting around by using the pipes, they walk fast down a corridor. Wires from a camera or a boom mike are visible on the left-hand side of the screen.
When Crabbe and Goyle pluck the chocolate cupcakes out of mid-air, you can see the shadow of the string on Goyle's face.
Early on in the movie, all of the sinks in Moaning Myrtle's Bathroom are turned on and overflowing. Yet later on in the movie one of the taps mysteriously never works. Even though it was working earlier.
When Ron and Harry run into Prof. Lockhart's office, after they just found out that Ginny has been taken into the Chamber of Secrets, Lockhart can be seen briefly in a mirror beside the door, waiting for his cue.
When Harry and Ron are in the Womping Willow tree one of the branches punches a dent in the roof of the car, but when the car drives away the dent is gone.