Continuity mistake: When Harry walks into Gringott's for the first time, if you look at the pattern on the floor he jumps forward about 5ft instantly. (00:21:10)
Continuity mistake: On the train, when Ron introduces Harry to Scabbers there are two Jelly Slugs lying on Ron's leg - one red-green and one yellowish-brown, but when Ron casts the spell on Scabbers both Jelly Slugs are red-green, and then it switches back again. (00:36:40)
Visible crew/equipment: After entering the Key room Harry says, "They're not birds... I'll bet one of them fits that door," and as they reach the levitating broomstick, at the bottom right corner of the screen, the shadow of the overhead boom mic moves on the floor. (01:58:20)
Continuity mistake: When Hedwig delivers the Nimbus 2000, as Harry catches it, the curve of the Nimbus' handle, the packaging and the way he grasps it all differ in the close-up and following shot. Then when Ron says, "Let's open it," he moves his goblet and plate to his left towards Hermione's goblet, yet in the next overhead shot his goblet is still on his right. (01:14:50)
Continuity mistake: During the scene with the troll, Hermione crawls over to where the sinks are and if you look at the floor you will see that there are only a few wet spots. In the very next shot, however, the whole floor is soaked (this occurs before the troll breaks all the sinks). (01:10:40)
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 we first see Hermione on the train, her tie is crooked. When she sits down, as Ron is about to change Scabbers yellow, her tie is straight and it remains that way while she is seated. When she leaves and turns around to tell Ron about the dirt on his nose, it is crooked again. (00:36:05)
Continuity mistake: When Quirrel collapses in the great hall on Halloween, his feet are quite far apart. Camera cuts to everyone rushing for the door, and Quirrel's feet are practically touching. (01:05:30)
Continuity mistake: When Fluffy drools on Ron, the amount of drool changes drastically. (01:51:30)
Continuity mistake: The amount of shirt that is untucked on Ron just after their exams is constantly changing from a lot to a little. (01:46:20)
Continuity mistake: When Neville first takes off during the 1st flying lesson, Ron covers his face with his right hand. When the shot changes, his hand is no longer over his face. (00:54:20)
Continuity mistake: On his first night in the castle, Harry is sat on a window ledge with Hedwig. In this shot, his feet go from resting on their sides and crossed at the ankles to flat and uncrossed. (00:48:50)
Continuity mistake: When we first see Harry drawing a cake in the dirt, his hand is in the shadow created by the sofa when lightning strikes. When we see a closeup of the drawing, his hand is in the light. (00:12:50)
Visible crew/equipment: When Dumbledore gives the order to the students to follow the Prefects towards their dorms after the troll came out from the dungeon, just a moment before Harry warns Ron that Hermione is unaware of what is happening, some light reflectors simulating lightning are reflected on the glass above the pathway where the students come from. (01:05:55)
Revealing mistake: When Ron is hit by the Queen during the chess game, the stunt double is visible on the horse just a few moments before he falls. (01:59:00)
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.