Visible crew/equipment: At the Quidditch match, after Hermione sets Snape's cloak on fire and someone shouts, "Fire! You're on fire!" there is a close-up of Harry with his hands gripping the Nimbus 2000. In this shot, the wire rods which are attached/taped to the broomstick, near Harry's fingers, that lead under both of his red sleeves, are entirely visible. (01:21:35)
Revealing mistake: Just after Harry gets back on his broom, the Slytherine seeker chases the snitch and he passes towers of (in order) Gryffindor, Hufflepuff, Ravenclaw, Slytherine, Gryffindor and Hufflepuff, on his left (this means Hogwarts is also to his left), when Harry flies up beside him. However, next shot as Harry slams into him, they still fly in the same direction, and the school is now on their right. Mistake with the background plate. (01:22:00)
Other mistake: In the Quidditch scene when Harry and the Slytherin seeker are chasing the snitch and are trying to bump each other off course you can see Hogwarts in the background. They pass Hogwarts twice while going the same direction. (01:22:05)
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)
Revealing mistake: When Harry is walking in the library's restricted section, you see the shadow of the library pillars on the bookshelf. If the light was coming from his lantern, there would be no shadow - the light is obviously from a spotlight elsewhere. (01:25:20)
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)
Continuity mistake: When Harry sneaks into the restricted section of the library with his lantern, he puts the lantern down on the ledge and the light doesn't even flicker, you can tell it's a sort of lightbulb, not a candle. (01:29:10)
Continuity mistake: In the scene in the library's restricted section, Harry puts down his cloak and then his lamp. The lamp is several inches away from the cloak. When Filch enters Harry grabs the cloak and suddenly the lamp is now on top of the cloak, causing it to fall. (01:29:10)
Revealing mistake: When Harry brings Ron to see the mirror of Erised, the green side of the invisibility cloak is seen when they are running to the mirror, near the door they entered from. (01:33:55)
Visible crew/equipment: Dumbledore tells Harry that he will move the Mirror of Erised and not to go looking for it. In the following shot Harry walks outside in the snow with Hedwig on his arm. In the close-up of Hedwig, the jess (strap) around his left leg is visibly tethered to Harry's sleeve. The inside of Harry's sleeve also has a strap that secures the sleeve around Harry's wrist. Hedwig is obviously not tethered in the next shot. (01:36:45)
Visible crew/equipment: Harry, Ron and Hermione run to Hagrid's hut to tell him that they know about the Stone and they knock on the door. As Hagrid opens the door in the first shot looking out, Harry stands behind his tape mark, and Hermione and Ron stand behind their shared black tape mark on the step. (01:38:35)
Continuity mistake: When Harry, Ron and Hermione are running to Hagrid's hut to tell him they know about the stone (just before they see Norbert) Hermione's hair is very long, but by the time they get inside his hut and take off their cloaks, her hair is much shorter. (01:38:40 - 01:39:25)
Visible crew/equipment: Harry, Ron and Hermione are in Hagrid's hut and the egg inside the cauldron in the fireplace starts rattling. In the shot looking up at Hagrid from behind the cauldron, at the top left of the screen in front of the fireplace, as Hagrid retrieves the egg a piece of equipment comes down then quickly goes back up and at the top in the center there is a small black film square that is not part of the fireplace. (01:39:35)
Continuity mistake: At Hagrid's hut, when he places the dragon egg on the table, the book lies right next to the cup and saucer. Yet in the shots looking up from the edge of the table, directly in front of Hagrid and the egg/Norbert, the book and cup/saucer are far apart for these particular shots. (01:39:50)
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.