From Movie Mistakes.com

• In the first film, we see that Susan Bones (the red haired girl) is sorted into Hufflepuff, however throughtout the Chamber of Secrets, Susan not only has her classes with the Griffyndors BUT is also wearing a Griffyndor tie.
• During the final moments in the Chamber, Harry overcomes Tom Riddle (Voldemort) when putting the Basilisk tooth through the diary. If you watch carefully, you will notice that Harry is keeping his right arm idle, as it has also been posioned by the tooth. He keeps striking the diary and finally closes it for one final attack on the cover. Right before he closes it, you see his left hand still poised in the air with the tooth, but as they cut to the closing of the book right away, they show Harry's left hand closing the book with no sign of the tooth. Now they cut back to Harry's face and his left arm is still up holding the tooth.
• When Harry and Ron are sitting in the hall doing work and Hermione comes up to them she swings her leg over and it is bare. Then 2 seconds later as she is sitting down she is wearing tights.
• When Harry first meets Dobby, Dobby is moving all about, yet Harry is just focused on one spot.
• In the scene where the Chamber finally opens, we first see Harry, Ron and Professor Lockhart from the front, and Harry and Ron are slightly behind Lockhart, and in a position to threaten and guard him. When you next see them a moment later from behind, Lockhart is behind them and they are in no position to guard him.
• When Harry and Ron crash into the Whomping Willow, the tree made a big hole in the back windshield. Later, when they are escaping in the car from the spiders, the hole is considerably smaller.
• When Harry first meets Dobby, Dobby is bouncing on the bed. There is a bulletin board of some kind with a Gryffindor flag thing on it. A couple of minutes later, the flaggy thing is still there, but the board itself is gone.
• When Harry is looking at the journal, a bright light appears right in his face and eyes, yet his pupils don't shrink.
• At the beginning of the scene near the end of the movie with Lucius Malfoy fuming at Dubledore 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.
• In the second to last scene when Dumbledore is talking to Harry and Ron, Ron's hair is a bit roughed and has a big cowlick. In the next shot, the cowlick is gone and both Ron and Harry's hair is neat.
• Colin Creevey is using an old Argus camera and flash unit. This is a flashbulb unit, with replaceable bulbs. You get one flash per bulb. Take a photo, and you get ONE flash and you must put in a new bulb. Yet we never see Colin put in a new bulb, he just flashes away on that same one over and over.
• When Ron and the Weasley twins come to pick up Harry from the Dursleys in the flying car, they fly over hundreds of houses. How is it then that we and Harry can hear the car when it is quite a distance away, but the people who live in the houses that the car flies over can't? The car isn't even invisible at the time.
• When Ron and Harry are escaping from the spiders in the flying car the passenger's side window that broke earlier hitting the willow tree is not broken, you can see Ron's reflection. A minute later you see that the window is broken again.
• Why doesn't Harry's scar hurt when he is with Tom Riddle, (Voldemort) like it did in the first film?
• After the basilisk is killed, and Harry talks to Dumbledore, the sword used is lying on the desk, covered in blood. Harry picks it up, and it's clean and shiny. Later, when it's back onthe desk, it's all messy again.
• The basilisk shown in the movie must be at least sixty feet long and 5-10 feet across. It would NOT be able to fit through pipes of any kind.
• 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.
• When Harry and Ron can't get through the barrier at Kings Cross, they go outside to the car. The outside of the station is St Pancras station and not Kings Cross.
• When Lockhart falls down the hole into the Chamber of Secrets, we hear him hit the ground a second or two later. When Ron and Harry jump down, not only do they take longer to get down, they also slide down the pipe, rather than fall straight down.
• In the scene where Professor McGonegal is teaching the class how to turn an animal into a goblet the writing on the blackboard at the front of the classroom is back to front.
• After Errol delivers the Howler to Ron, he flies very close to Hermione's head, the wire attached to the Owl is clearly visible as it passes her hair. It looked like it actually hits her in the face.
• In the chamber, as Harry crawls back to Ginny after killing the Basilisk, there is a pool of black ink in the water just past Ginny's head. It is only as the scene continues that Harry stabs the diary and it leaks all the ink - causing a trail down to the water, where the pool of black ink is.
• In the first film, Harry's family clearly live in a small close with trees behind their house - see owls delivering mail. The opening of COS shows Privet Drive to be in the middle of a large housing estate with houses to the rear.
• When Harry is in Diagon Alley and Hermione fixes Harry's glasses, students are not allowed to perform magic out of Hogwarts or off the Hogwarts Express, so Hermione shouldn't have been allowed to do that magic, and she is a very law-abiding person.
• After Harry has been told that he only has a minute to live, he seems to have enough strength to put the tooth right through the diary and sit up and tell Ginny what to do.
• During the filming of The Chamber of Secrets, Emma Watson broke her wrist. In the trailer, when Harry, Ron, and Hermione are drinking the polyjuice potion, you can see the cast on Emma's wrist sticking out of her robes.
• Notice as Harry walks into his room his hair is flat. They cut to Dobby for a bit and they return to Harry, but this time his hair is a bit messy.
• The writing on the wall is is bright red from the side shot, but when you see the message full on, the writing has turned brown.
• A big change for us is that Tom Riddle's memory disappeared before Fawkes cried on Harry's wounds.