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)
Revealing mistake: At the zoo, Harry looks to his right, at the 'bred in captivity' sign of the Python. In this close-up, the green plant that is below, to the side of the exhibit sign, is protruding out past where the glass supposedly is, yet this is BEFORE the glass disappears. As a matter of fact, this plant, nearest the exhibit sign, changes its position considerably throughout these shots. (00:06:50)
Factual error: When Harry talks to the snake, the camera shows a close up of the snake, and it blinks. Snakes don't blink because they lack eyelids. (00:06:50)
Continuity mistake: At the snake exhibit, while Harry speaks with the Python the snake's body lies curled over a long wood log and a large rock. In the shot from outside the Python's exhibit, after the glass pane disappears Dudley falls over the rail and the snake is gone, not positioned differently or blocked from view, but altogether gone. In the next overhead shot inside the Python's exhibit, while Dudley falls the snake is back lying on the log and rock, in the very same position as in the previous shots. (00:07:10)
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: When trapped in the Python exhibit at the zoo, Dudley stands up, presses his wet palms on the glass and nervously bangs on the glass while dripping plenty of water on it. However, in the next two shots after yelling, "Mummy, help! Help me!" the glass is all dry and just wet beneath his palms. (00:07:50)
Continuity mistake: When Harry lifts the mail from the floor at the Dursleys, the Hogwarts envelope flap is sealed shut and it has the red wax seal at the tip. Yet in the kitchen after he hands Vernon the other letters, Harry holds the Hogwarts envelope and though the red seal is on the back, the flap itself is NOT sealed shut. The wax seals differ in both shots as well. (00:08:45)
Continuity mistake: The size and style of the writing on the envelope from Hogwarts at the starts changes depending on the shot. When Uncle Vernon picks it up, the word "stairs" is a fair way from the edge of the envelope. When he flips it over and you get a close up, the same word is a lot closer to the edge. (00:08:50)
Other mistake: When Harry receives his first letter in the mail, Dudley snatches it from him before he has a chance to open it. Then he gives it to his father (Uncle Vernon), and Uncle Vernon says "Who'd be writing to you?" If you watch Dudley carefully, you can see him mouth what Uncle Vernon is saying. (00:08:54)
Continuity mistake: In the Dursley's kitchen, as Harry walks by, on the table is Vernon's cereal bowl between the coffee pot on his right and OJ on his left, plus the tea pot is in the center of the table. In the close-up as Vernon says, "Yours? Who'd be writing to you?" the cereal bowl is missing and the tea pot is near his OJ. In the next close-up of the letter, the cereal bowl is back and the OJ is now on Vernon's right, near the coffee cup. (00:09:05)
Visible crew/equipment: Harry gives Vernon two letters and he walks to the other end of the table to open the Hogwarts envelope. When Dudley grabs the letter, as he shouts, "Dad look! Harry's got a letter!" under Dudley's chair, to the left, the black tape mark is visible on the carpet covering. The carpet is covered over near Dudley's chair and on the other side of Vernon, near the black mat. In the close-up of the letter as Vernon holds it, the carpet is back to normal near Dudley's chair. (00:09:10)
Revealing mistake: In the shots facing Vernon, he holds the first Hogwarts letter while sitting at the table. Though in the close-up of the letter as he holds it, it is not Richard Griffiths' hand, that is shown with bitten up fingernails, holding the envelope. (00:09:15)
Revealing mistake: When Uncle Vernon picks up the letters for Harry and tears them up, you can see cuts in the envelopes, in order to make it easier for him to tear all of them at once. (00:09:40)
Continuity mistake: When Uncle Vernon is nailing a small piece of board to the mail slot, he nails it angled slightly to the right. When Harry's letters start pouring in and the small piece of board is ripped from the door, it is angled slightly to the left. The actual wood boards differ as well. (00:09:55 - 00:12:00)
Revealing mistake: While Vernon kneels as he tosses Harry's Hogwarts letters into the fire, the interior solid back of the fireplace is visible behind the flames. On Sunday, the first letter (from the fireplace with a CG back) hits Vernon in the face as it flies by. In the following two shots of the fireplace, as more letters fly out, there are now large holes at the interior back of the fireplace that are visible. As the letters pour down from above, air is blown through these holes to blow the letters out and about. (00:10:20 - 00:11:20)
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)
Other mistake: Near the beginning, when the Dursleys' house begins shaking, none of the objects fall nor do the curtains shake. (00:11:00)
Revealing mistake: In the flying letters scene, when the letters are coming through the chimney, if you look closely when Harry catches one you can see that the letters are all photocopied because the letter seal is not raised. (00:11:05)
Audio problem: When the letters come flying out of the fireplace, Dudley yells, "Please, make it stop." If you look at his lips during this time, they're not moving. (00:11:25)
Continuity mistake: When all of the letters are arriving from Hogwarts at the Dursleys' house, it is obvious that the ones flying around the room are just single pieces of paper and not thick envelopes containing letters. (00:11:25)
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.