Continuity mistake: At the beginning, Dumbledore is taking out all of the street lights, from one shot, the street light closest to Dumbledore on HIS right is still lit, but in the next shot, which is a close up of him taking the light, and shots later, the street light is out and he never "took" the light from that one. (00:01:05 - 00:01:55)
Continuity mistake: The very first scene where you see Hagrid on his motorcycle, he comes down and, from his position, you can tell he's gonna land way past the first house on the set. The next shot you see him touch down in front of that house. (00:02:15)
Continuity mistake: When Hagrid steps off the motorbike while holding young Harry, the Dursley's house (with a small bush to the right of the front door) is visible directly behind McGonagall. The house is fully attached on both sides to homes with a one car garage. Yet after Vernon asks, "Who'd be writing to you?" in the shot of the owl dropping the letter which flies through the front door mail slot, the house to the left of the Dursleys (when facing the front) has a two car garage, is not attached to the Dursley's house and is quite a bit away from the Dursley's house as well. Then in Chamber of Secrets, the house to the right (when facing the back) is attached once again in the same manner as at the start of this film with young Harry. (Visible on DVD widescreen.) (00:02:25 - 00:09:25)
Continuity mistake: Dumbledore, McGonagall and Hagrid leave baby Harry on the Dursley's doorstep. When the shot opens in the first close-up of the baby, as Dumbledore places the envelope on the blanket, though a good portion of his forehead is visible, the scar is not there. In his next close-up, it is very apparent, due to the position of the scar, that part of it should have been visible on his forehead in the previous close-up. (00:03:40)
Continuity mistake: The angle/shape of Harry's scar changes dramatically in quite a few shots throughout the film. Two examples being, the morning of Dudley's birthday, when Harry opens his eyes in the close-up, while lying in the cupboard; and later, when his hair is blown back in the close-up, as Voldemort's 'spirit' starts to rush through him, while he holds the Stone. (00:04:05 - 02:13:25)
Continuity mistake: On Dudley's birthday, when Mrs Dursley wakes Harry and he sits up in bed, a cloth and hand brush are hanging next to him on the inside of the cupboard door. When Dudley shoves Harry back into the cupboard, there is now a cloth and a dustpan, but no brush. When the family arrive back from the zoo, and Mr Dursley locks Harry in the cupboard, there is no cloth, brush or dustpan. (00:04:10)
Continuity mistake: The morning of Dudley's birthday, Petunia wakes Harry and the next shot shows Harry in the cupboard with the shelves behind him. When Dudley runs downstairs and pushes Harry back into the cupboard, things inside are positioned differently. This also occurs in the two shots when Harry plays with the small plastic figures inside the cupboard and then when he peeks out as Vernon attaches the wood to the mail slot. (00:04:20)
Continuity mistake: After Dudley shouts, "Wake up, Potter! We're going to the zoo," he pushes Harry back into the cupboard and runs into the kitchen. When he opens the door, the kitchen floor is visible and there are no floor mats. Yet, once Harry walks into the kitchen there is a taped black mat on the floor at the door and another black mat at the counter, under the stools that were visible in the previous shot, when Petunia bangs on the cupboard door. (This does not refer to the reddish mat Dudley stands on.) (00:04:35)
Continuity mistake: When Harry walks into the kitchen, Petunia tells him to take care of the breakfast and two of Dudley's presents, the blue design and the yellow, are visible on the couch. When Petunia asks, "Aren't they wonderful darling?" the camera pans the presents, including the ones from the earlier shot. When Vernon says, "Well some of them are quite a bit bigger than last year's," the two presents are again visible. The position of these gifts differs in all shots. (00:04:50)
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: 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: When Petunia covers Dudley's face, her hands alternate from right over left to left over right. (00:05:30)
Continuity mistake: When Vernon says "I'm warning you now boy" between shots his coat changes position. (00:05:35)
Continuity mistake: In the scene where Harry and the Dursleys visit the zoo, the first shot you see of the snake, he is lying on a bare rock with only moss on it. He is also lying with his head facing the window, and his body is curving to the right. However, after Dudley walks away and the shot returns to the snake, his body is curving to the left and now there are plants directly behind him where there was only moss before. (00:06:00)
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: 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)
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.