Revealing mistake: When Uncle Vernon picks up the letters for Harry and tears them up, you can see cuts in the envelopes, in order to make it easier for him to tear all of them at once.
Other mistake: In the scene near the end of the movie where Harry boards Hogwarts Express to go home, Hagrid stands on the platform and waves goodbye. He is wearing a coat and looks huge, as usual, but you can see when he waves, that his hand seems rather small compared to the rest of his huge frame.
Continuity mistake: When the three heroes first go into the forbidden corridor and are spotted by Filch's cat, they run away, through the large horizontal corridor which must be at least 40 metres long (since we are given two long shots of it and neither of them connects by any physical object). They hide behind a wooden door which they close afterwards. When Filch looks in the other end of the corridor then leaves and we hear a very silent click, Hermione says 'Filch is gone.' The door they're behind does not seem to be open at all, and even if it is open a bit on the right side, Hermione's on the left side of it so she can't see whether Filch is gone or not. And can you hear a silent click of a closing door from forty metres?
Visible crew/equipment: During the chess game when the queen drives her sword through Ron's knight, if you pause it just right you can clearly tell that it's a stunt double falling off the horse.
Visible crew/equipment: When Oliver Wood is explaining Quidditch to Harry, he struggles to put the Bludger back in the trunk. You can see a rod under the trunk lift and shake it, so the Bludgers appear to be moving on their own.
Continuity mistake: When Harry and Ron rush into the bathroom after the troll to rescue Hermione, they push open the closed doors, but wouldn't they be open from when the troll went in there moments before? The doors can't have closed after the troll as when Harry and Ron push open the door, it stays open.
Continuity mistake: When Professor McGonnagall is talking to the children before their sorting, you can tell when the camera is on her that the lighting is candle-like (note the flicker). When the camera cuts to the children, there is no flicker (check out Hermione).
Continuity mistake: When Harry and Ron are sat on the train, Ron's hair begins with a centre parting, but soon changes to a side parting.
Continuity mistake: When Hagrid drags the Christmas tree into the castle, in the first shot there is no snow on his hair, although he has supposedly been outside for some time. When the camera angle changes his hair is now covered with snow.
Other mistake: When Harry, Ron and Hermione rush to Hagrid after the end of the year exams, and Harry is saying something that ends with, "Why didn't I see it before?", Hermione is mouthing part of that line.
Continuity mistake: Near the start, Percy says "Boys upstairs and down on your left, girls the same on your right". However, after the three heroes have been to investigate Fluffy and they go back to the dorms, when they reach the top of the stairs Hermione goes to the left and the boys head right.
Visible crew/equipment: Hagrid surprises Harry with the box of birthday cake and Harry opens the box. In the close-up of the cake, as Harry holds the box with his left hand and moves the blue string with his right hand, a crew member's finger is visible under the box, over Harry's left shoulder. Obviously it is someone holding the box underneath, while Harry holds it with one hand.
Continuity mistake: After Harry opens the package with the invisibility cloak, he stands near the red chair and when he says, "I don't know, some kind of cloak," the cards on the back board are visible. As Ron says, "Well let's see then. Put it on," he holds a Bertie Botts box in his left hand and has his right hand in the box. Next shot facing Harry, Ron visibly holds the box with his right hand and the positions of the pillow, cushion, envelope and letter differ on the red chair. As the next shot opens facing Ron, he holds the box in his left hand and his right hand is in the box. Then in the next shot facing Harry again, the bottom card on the back board is gone.
Continuity mistake: During the scene with the troll, Hermione crawls over to where the sinks are and if you look at the floor you will see that there are only a few wet spots. In the very next shot, however, the whole floor is soaked (this occurs before the troll breaks all the sinks).
Revealing mistake: In the shot where Harry is running through the wall for platform 9 3/4, the camera pans down slightly and you can see a rather stiff and fake looking Hedwig wobbling in her cage.
Continuity mistake: When Hedwig delivers the Nimbus 2000, as Harry catches it, the curve of the Nimbus' handle, the packaging and the way he grasps it all differ in the close-up and following shot. Then when Ron says, "Let's open it," he moves his goblet and plate to his left towards Hermione's goblet, yet in the next overhead shot his goblet is still on his right.
Continuity mistake: When we first see Hermione on the train, her tie is crooked. When she sits down, as Ron is about to change Scabbers yellow, her tie is straight and it remains that way while she is seated. When she leaves and turns around to tell Ron about the dirt on his nose, it is crooked again.
Continuity mistake: When Harry receives his letter from Hogwarts, he opens it and it looks like one letter standard size. When the shot changes, it looks like a card (but we saw Harry unfold it). And when they are in London, right before they enter The Leaky Cauldron, it's standard size again but now there is more then one letter. Notice the folded left hand corner.