Revealing mistake: After Madame Hooch takes Neville to the hospital wing, Malfoy takes the Remembrall and mounts his broomstick, on a convenient protruding metal rod. Then when he comes around, though the students step away and watch as he begins to fly off, many of their eyes are not level with Malfoy, as he is merely edited into the end of this shot. (00:57:50)
Continuity mistake: After the final confrontation Harry has scratches on his face, particularly one on his chin. When visited by Prof Dumbledore the chin scratch is barely there but in the scene in the refectory it is back and appears larger than before. (02:14:00 - 02:17:50)
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 Madame Hooch leaves with Neville to go to the hospital wing, Seamus holds his broomstick with the bristles on the ground as Malfoy ridicules Neville. However, as Malfoy steps on his broomstick and then flies away, the bristles on Seamus' broomstick are now at the top. Then when Harry takes off on his broomstick, the bristles on Seamus' broom are on the ground once again. (00:57:30)
Continuity mistake: At the hut on the rock, Harry lies on a grey blanket spread on the ground between the fireplace and the couch. When Hagrid bangs on the door, Harry quickly jumps up and hides, leaving the blanket behind. Though the blanket is visible on the ground, in some shots, it is however, gone in the three wide shots with the Hagrid body double, as he zaps a tail on Dudley, then as all the Dursleys run upstairs, as well as the shot after Hagrid says, "We're a bit behind schedule, best be off." (00:12:55)
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)
Continuity mistake: When Dudley falls into the water in the Python's exhibit, his clothes are completely soaked. Yet when Dudley yells and bangs on the glass, in the shot facing Petunia as she screams, it is quite apparent that Dudley's sweater/jumper is not entirely wet. The wet hair at the back of his head lies flat in the second shot as well. (00:07:15)
Continuity mistake: At the hut on the rock, after the camera pans sleeping Vernon and Petunia, there's an overhead shot of Harry and Dudley downstairs. In this shot the hanging light, its switch and cable are visible over the stairs. Note, the cable switch hangs down low, parallel to the droplight. However, when Vernon and Petunia are at the stairs, and she turns on the light, the cable and switch are now perpendicular to the droplight, raised higher up parallel to the wood above. Seen in following shots as well. (00:12:25)
Continuity mistake: After Seamus' disaster with the rum spell, the mail arrives. Ron catches his mail, then holds the Daily Prophet in his right hand and the letters in his left. Yet in the next close-up he holds the Daily Prophet in his left hand and the letters in his right. (00:54:05)
Continuity mistake: As the students board the Hogwarts Express to go home, the cuts on Harry's face, particularly near his eye, differ in severity during this scene depending on the camera angle in a given shot. (02:21:55)
Continuity mistake: In the scene where Harry, Ron and Hermione come with Filch to Hagrid to go into the dark forest, watch the lamp Hagrid sets down. Filch says something like "A sorry lot this, Hagrid". Then the shot cuts to Hagrid (look at the lamp) then to Filch again who says "Oh good god man, you're not still on about that bloody dragon, are you?" then to Hagrid again. The lamp changes position between these shots. (01:43:00)
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 Ron and Hermione run to see McGonagall, there is an hourglass, a large book and a candle all sitting directly on top of her desk. Their positions change, especially in the shot facing McGonagall, when she says, "I'm afraid Professor Dumbledore is not here...," not only is there is another small book next to the candle, the hourglass now sits on the large book. The long iron inkwell in the middle of the desk has also been turned around. (01:51:45)
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)
Revealing mistake: During the Quidditch game, none of the players have shadows on the ground. (01:16:20)
Continuity mistake: When Wood takes Harry out to see the Quidditch balls, he sets down the bat right against the box holding the balls. When Wood goes to open the box, you see the grass where the bat should be. And then he picks up the bat, which is not where it was originally placed. (01:01:30)
Revealing mistake: After being dropped by the troll in the girls' lavatory, Harry then scoots backwards on his backside and it is apparent that he is wearing knee pads under his pants. (01:11:40)
Continuity mistake: In the overhead wide shot, before Madame Hooch says, "Good afternoon, class," as she approaches, each student stands directly behind the bristles of their broomsticks, yet in the next shot as the camera pans their feet, they stand between the bristles of their broom and the neighboring broom. (00:55:00)
Revealing mistake: At the start of the overhead shot as the troll's club smashes the cubicles, while Hermione lies on the ground, the top beams of all the cubicle walls and the front door of the second cubicle are all visibly pre-broken before the club slams into them. (01:10:15)
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.