Continuity mistake: While Harry and Hagrid stand beside Hogwarts Express, in the shot looking up at Hagrid as he takes the album out of his pocket, his thumb is on the back cover as he begins to say, "This is for you," yet in the next shot of Harry taking the album, Hagrid's thumb is on the front cover. (02:22:05)
Continuity mistake: When Vernon says "I'm warning you now boy" between shots his coat changes position. (00:05:35)
Continuity mistake: On the Hogwarts Express, Hermione first watches as Ron tries to turn Scabbers yellow, she then joins them inside before she actually leaves and in these consecutive shots, her tie differs. (00:36:30)
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. (00:25:30)
Continuity mistake: In the scene where Neville lost control of his broomstick during flying lessons, we see a shot of him going straight towards the castle wall (this is the first time he sees the wall). When you see him zooming towards the wall there are many windows. However, when the shot changes and Neville actually bumps into the wall with his broomstick, there are no windows seen. (00:56:35)
Continuity mistake: Malfoy holds Neville's Remembrall and as he begins to deride Neville, Hermione stands between Harry and Seamus. Yet in the next close-up Seamus is right next to Harry and then inches over a bit as Harry moves closer to Malfoy. (00:57:35)
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? (00:06:30)
Continuity mistake: When Petunia covers Dudley's eyes with her hands and leads him to the table on his birthday, Dudley has a red mark on his left cheek that looks like Petunia's lipstick. The mark is gone as Petunia removes her hands. (00:04:59)
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. (01:18:20)
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. (00:48:25)
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. (01:22:25)
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. (00:12:35)
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. (01:22:05)
Continuity mistake: Right after Hagrid tells the kids about playing music to Fluffy, they run into Professor McGonogall's classroom. Right before they run in, you can see a candle on a pole to the left of her desk. When they run up to her desk, it isn't there anymore. (01:51:35)
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. (00:33:30)
Continuity mistake: At Platform 9 3/4, when Mrs. Weasley says, "All right, Percy, you first," as Percy walks in front of her she holds her bags in her left hand. Yet in the next shot as Percy still walks in front of her, she holds her bags in her right hand. (00:32:25)
Continuity mistake: In the scene where Harry, Ron, and Hermione are walking towards Hagrid after their exams, Hermione is carrying a brown satchel in her right hand. Then Harry says, "Why didn't I see it before!" and they run. While they are running she does not have the satchel in either hand, her empty left hand is visible in a few frames as she swings her hands at the side of her body. Then when they get to Hagrid she has the satchel in her left hand. (01:46:40)
Continuity mistake: In the scene where Hagrid sees the unicorn blood in the dark forest, take a good look at the silver puddle of blood. When Hagrid takes some on his fingers, the puddle changes shape and size. (01:44:30)
Continuity mistake: When Dudley is told how many presents he has, and he asks, "How many are there?" a cut to him shows that his fringe is a bit untidy. Scene cuts to his Dad. When it cuts back to Dudley, his fringe is much neater. (00:05:04)
Continuity mistake: At his home, when Vernon says, "Fine day, Sunday. In my opinion best day of the week. Why is that Dudley?" in the close-up his arms are crossed right over left. In the next wide shot, as Harry answers, Vernon's arms are crossed left over right. Yet in the next close-up, his arms are crossed right over left again. (00:10:40)
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.