Audio problem: During the scene with the "flying" letters Harry gets one letter and tries to go in his "room", but Uncle Vernon grabs him. When Harry yells "They're my letters," not only his mouth doesn't move but his voice sounds much deeper than in the rest of the movie (it sounds more like his voice in Chamber of Secrets). This indicates that this line was overdubbed long after the main filming, when Daniel Radcliffe's voice has already begun to change. (00:11:55)
Audio problem: When the troll is about to swing at Harry for the third time with his club, Ron levitates his club in mid-air. When the club falls, it hits the troll's head with a loud thunk, then falls to the ground. But you never hear any sound of the club hitting the ground. (01:08:44)
Audio problem: When Harry, Ron and Hermione are running away from Mrs. Norris (the cat), Harry yells, "Quick hide through that door!" and if you look close enough you can see that his mouth doesn't move when he says that. (01:01:45)
Audio problem: When Hedwig flies from Harry's arm, or owls deliver mail, you hear the sound of wingbeats. However, owl's feathers are specially adapted to allow for completely silent flight. See here for an explanation. http://www.owlpages.com/articles.php?section=Owl+Physiology&title=Feathers. (00:53:45)
Audio problem: When Dudley is complaining about the number of birthday presents he got, his mouth doesn't match with what Dudley is yelling. His line is badly dubbed. (00:05:00)
Audio problem: In the scene at the very end of the movie when Hermione and Harry are getting on the train, Hermione says, "Feels a bit strange going home, doesn't it?" Harry responds, "I'm not going home, not really." If you look closely, Harry says the words and his mouth moves. Then the camera changes angles and Harry mouths the same words once more, but no sound comes out. (02:23:15)
Audio problem: When the letters come flying out of the fireplace, Dudley yells, "Please, make it stop." If you look at his lips during this time, they're not moving. (00:11:25)
Audio problem: In Gringotts Bank in Diagon Alley as soon as it comes to the scene with the cart Harry and Hagrid are sitting in with the goblin, when they stop and the goblin says "vault 687" if you look close enough, you can see that his mouth isn't actually moving at all. (00:23:10)
Audio problem: Amidst the flying letters, as Vernon begins to chase Harry, when he yells, "Give me that!" and continues with, "Give me that letter!" his lips do not move when visible on screen. (00:11:40)
Answer: Petunia or Vernon would have ran to a zoo employee and told them what happened. The employee would then open the door to the snake exhibit letting Dudley out.