Continuity mistake: When Hermione is confronted with the troll in the lavatory she backs up and runs into the cubicle. In the overhead shot as she locks herself in, the pattern on the floor that is visible in front of the next cubicle is different than the previous and following shots. (01:10:10)
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). (01:10:40)
Continuity mistake: As Hermione hides under the sinks the troll smashes the front basin of the second sink on the right, leaving its faucets and surrounding sinks intact. Though in a view of the entire lavatory when the troll holds Harry upside down, from behind Hermione, the entire second sink, its faucets and trim are entirely missing and the entire fourth sink, which was never touched, is as well, both are gone. When the troll is knocked out, as Hermione stands up and moves toward Harry and Ron, though the faucets of the second sink are back, the front basin of the second sink is visibly broken again just as in the original shot, and the entire fourth sink is still inexplicably gone in this shot. (01:10:45 - 01:11:45)
Continuity mistake: When Harry, Ron and Hermione are in the lavatory with the troll, the scattered wood pieces of the damaged cubicles change positions in consecutive shots throughout. For example, just before and after the troll drops Harry, in the close-ups of Harry the wood differs from the consecutive overhead shots. (01:11:35)
Continuity mistake: When Hedwig comes flying in delivering the broom, she drops it at an angle, but when Harry catches it it's completely horizontal. (01:11:50)
Continuity mistake: As Hermione walks toward the troll, just before she asks, "Is it dead?" the long towel hanging from the rod beside the sinks is visibly straight. When Harry lifts his wand with troll booger/bogey, in the shot of him as he grimaces and groans, the towel on the rod is visible behind him and it's messy. Yet when McGonagall, Snape and Quirrell walk in, the long towel is straight again. (01:12:00)
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. (01:14:50)
Continuity mistake: The knotted tie string on Harry's Quidditch robe changes from extremely loose, to snug, to loose and so on, even in consecutive shots, from the time Harry speaks with Wood before they fly onto the pitch and throughout the match. This occurs with other members of both teams as well. (01:15:40)
Continuity mistake: Just after Slytherin scores ten points, Flint and his teammate surround Angelina. When one grabs the quaffle from Angelina, they fly past the second Gryffindor tower - we see the first Gryffindor tower, two staff towers (grey/black), Slytherin tower and the second Gryffindor tower behind them - then Angelina covers her head with her arm. However, in the next shot she actually crashes into the tower that she already flew past. (01:16:30)
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. (01:17:40)
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: Just after Flint leaps over the Weasley twin, in the next shot Flint tosses the ball forward - toward the hoop - with the school to his left (viewer's right) and Ravenclaw, Slytherine, Gryffindor and two staff (black/grey) towers behind him, to his right. However, in the next shot as the ball flies through the hoop (close-up), it is actually the hoop at the other side of the pitch - we see (in order) the Slytherine, Ravenclaw, Hufflepuff and Gryffindor towers to the left of the screen. (01:19:30)
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: 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: In the Quidditch scene, when Madame Hooch steps onto the field she says, "Now I want a nice, clean game," and while she's looking up, her Hogwarts patch is on the left side of her robes. At the end of the Quidditch game when she says, "Gryffindor wins," her patch is on the right side of her robes. Interestingly, the shot of Madame Hooch is a flipped shot, yet it was pasted onto the scenic background, which is not flipped, but properly positioned. (01:23:00)
Continuity mistake: When Harry raises his hand with the captured Snitch, his back faces the Slytherin and Hufflepuff towers which are opposite each other, with the hoops between them. However, in the next shot he is now facing that side of the pitch, as other Quidditch players appear from that side. (01:23:25)
Continuity mistake: In the scene straight after the Quidditch match where Harry, Hermione and Ron are telling Hagrid about their suspicions of Snape, you will see all three are wearing scarves tied in different ways. When there's a change of shots, the creases in the scarves change. This happens mostly with Hermione's. (01:23:55)
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. (01:25:10)
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. (01:27:50)
Continuity mistake: When Harry is on his way to the restricted section of the library under his Invisibility Cloak, we see a shot of his hand holding the lantern (the first shot in the scene). You can see the end of his jumper. In the next shot, however, the end of his jumper is under the cloak, and it had no time to get there between shots. (01:28:30)
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.