Continuity mistake: At Kings Cross, just as Percy Weasley is about to run through the wall onto platform 9 3/4, we see a man and a woman walking past him, but when the shot changes direction, they have both disappeared. (00:31:15)
Other mistake: During the Quidditch match, Hermione looks in her binoculars up to where Harry is. If you look at the reflection in the binoculars, you can see Harry flailing around. She then looks over at Snape, but when we see the reflection in the binoculars, we still see Harry, as if she was looking up towards him. (01:20:40)
Continuity mistake: When Harry, Ron, and Hermione are heading to Hagrid's hut after discussing the stone, Harry says 'Why didn't I see it before?' his tie is all the way over to his right side, then when they are talking to Hagrid his tie is straight in the middle. (01:46:00)
Revealing mistake: After Hagrid gives Dudley a tail, watch him in the background while Dudley runs around. It's easy to see it's not Robbie Coltrane but a body double (Martin Bayfield), who is also in a few other shots at the hut. (00:17:45)
Continuity mistake: When Petunia covers Dudley's face, her hands alternate from right over left to left over right. (00:05:30)
Visible crew/equipment: When Malfoy steps onto his broom while holding Neville's Remembrall, the actor's two curved chalk marks can be seen on the ground near Hermione's feet, and just as Harry steps backward we can see the actor's chalk marks on the ground near his feet. (Fullscreen version). (00:57:30)
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)
Continuity mistake: During Neville's ill-fated flying lesson, as he approaches the statue from behind, we see that the statue is placed atop a round turret with alternating levels of bricks around the top of the turret. Neville's robe then snags on the spear of the statue. When we see the front of the statue as Neville falls, the statue is now standing atop a flat, level section of wall. (00:56:55)
Continuity mistake: The first wand Ollivander gives to Harry has a long slim handle with 5 or so grooves carved at the wand end. When Harry puts the wand down, the grooves are even with the binding of the book lying there. When Ollivander hands Harry the second wand, look to the left by the book and you can see that not only has the wand been moved, it is actually a different wand with a shorter, wider handle and only one groove at the wand end. (00:26:10)
Continuity mistake: When Harry and Ron are on the train to Hogwarts, we see a shot of the sweets Harry has bought. Note the position of Harry's sweatshirt and that the collar shows no defects. A couple of shots later, the sweatshirt appears to have moved and more of his left neck is showing. A few shots later, when Hermione arrives, there is another shot of the sweets arranged very differently. When Hermione points her wand at Harry, there is a small tear in his sweatshirt collar. (00:33:53)
Visible crew/equipment: In the scene where Harry and Ron come running down the hallway and see the troll for the first time, you can see a floodlight at the top of the frame illuminating the area where they stop and hide. (01:09:30)
Continuity mistake: When Harry lands and gets off his broom after catching the Remembrall during the flying lesson, he starts running over a shadow of the castle. But in the next shot, it has either been shot at a different time of day, or he has gone past the shadow and has run further up the path. (00:58:55)
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)
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: In the chess scene, when Ron falls off the horse, he is lying on his side on the floor with one knee bent so that one foot is higher than the other. When Harry and Hermione run over to check on him, one foot is on top of the other. (02:05:15 - 02:06:00)
Visible crew/equipment: When Oliver Wood is explaining Quidditch to Harry, he struggles to put the Bludger back in the trunk. You can see a rod under the trunk lift and shake it, so the Bludgers appear to be moving on their own. (01:05:55)
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)
Continuity mistake: When Harry and Ron are sat on the train, Ron's hair begins with a centre parting, but soon changes to a side parting. (00:32:59)
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)
Visible crew/equipment: Hagrid surprises Harry with the box of birthday cake and Harry opens the box. In the close-up of the cake, as Harry holds the box with his left hand and moves the blue string with his right hand, a crew member's finger is visible under the box, over Harry's left shoulder. Obviously it is someone holding the box underneath, while Harry holds it with one hand. (00:14:10)
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.