Revealing mistake: During the chess game the Queen attacks the Knight's horse that Ron sits upon. In this close-up just before the spear actually hits the side of the horse, the large hole, used to aid the stunt, is visible in the belly of the horse between the stirrups towards the hind legs. This hole is not in previous shots. (Visible in fullscreen DVD.)
Continuity mistake: In Harry's first Quidditch match, Gryffindor scores the first goal, then we see the commentator announce the goal and change Gryffindor's score to ten - making a goal worth ten points. Soon after, Gryffindor score again. We then see Slytherin score a goal, then another. They then score a third goal, and the commentator changes the score to 20 all, but it should be 20/30.
Continuity mistake: Mr. Ollivander says, "I wondered when I'd be seeing you Mr. Potter," and as he climbs down the ladder Harry is standing at the middle of the desk, away from the corner. In the next shot as Mr. Ollivander searches for a wand case Harry stands at the corner of the desk. However, in the next shot Harry stands at the middle again.
Continuity mistake: When Percy leads the group of first years back to the Gryffindor dormitories for the first time, at the stairs he says, "Keep up please and follow me. Quickly now, come on." In this wide shot Harry and Ron run up the stairs first and Hermione is at least six people behind them. Yet, in the next close-up she is with them at the front, right behind Percy.
Revealing mistake: When Harry, Ron and Hermione are making their way back to the Gryffindor common room, Ron says 'She knows more about you than you do', to which Harry replies 'Who doesn't?' The "solid brick" staircase moves so that they have no place to go but the third floor forbidden corridor. If you pay attention to the staircase it can be seen that there is a good gap between the staircase and the one it connects to when it moves and that when Harry, Ron and Hermione run off the staircase to the corridor the staircase made of "solid stone" does an awful lot of shaking.
Continuity mistake: in the fist chess scene (Harry vs Ron), when we first see the board, Harry's knight is at D3. As Hermione walks down the aisle Harry announces, and plays NE5 and we see the knight move itself to E5. Then as Hermione approaches the board, we again see the pieces and the knight is back on D3 again. Also, the black (red) pawn at H5 has mysteriously disappeared, or maybe retreated to G6.
Continuity mistake: During the Quidditch match, after Harry pulls out of the dive, the shot changes. He is now flying completely in the opposite direction as he crosses the halfway line. He continues forward to what should be near the end of the pitch, but when he falls to the ground he is back at the halfway line.
Continuity mistake: When Harry is talking to the snake in the zoo we first see the snake. Then in the next shot we see Harry with Dudley behind him at the other exhibit window and Dudley turns around and then back again. Then there's another shot of the snake, then Harry again and behind him Dudley turns around again and notices Harry, but why then and why not the first time he turned?
Continuity mistake: Just as Harry runs towards the wall to reach Platform 9 3/4, it is obvious that the owl in the cage is now a stuffed one.
Continuity mistake: In the quidditch scene, when Hermione sets fire to Snape's robes, the flame lights at a fold in the material, but in the next shot it's moved away from the folds. In fact, there are no signs of it having ever being burned there.
Revealing mistake: While sitting on the couch in the hut on the rock, Hagrid asks, "Did you ever make anything happen, anything you couldn't explain? When you were angry or scared?" In the shot when he's getting up off the couch, it is blatantly apparent that the fingers on his left hand are latex in this shot. The hands are visibly artificial in other shots as well.
Continuity mistake: While Harry lies on the floor at the hut on the rock, with his finger he draws birthday candles atop the birthday cake. When Dudley's watch hits 12:00, in the next shot of Harry, the candles visible on the floor do not resemble the ones in the previous and following shots.
Revealing mistake: Has anyone else noticed that Ron's rat, while on the Hogwarts Express at the beginning, is at one point fake! When he is explaining his spell to Harry, it comes into shot and is completely still, it moves like it is a toy.
Continuity mistake: When Vernon says "I'm warning you now boy" between shots his coat changes position.