Revealing mistake: When Harry and Hagrid are at Gringotts and are pulling up in the cart to Harry's vault, in the overhead shot you can see the various levels bellow them. In the next shot in the lower left corner you can see what appears to be blue screen just bellow the walking surface.
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.
Continuity mistake: When Hagrid and Harry step out the back door of the Leaky Cauldron, in the overhead shot there are many glass bottles on the floor, between the barrels. However, after Hagrid says, "Welcome Harry, to Diagon Alley," as they walk out the barrels are visible behind Hagrid and there are no glass bottles between the barrels. The stone ground differs in both shots too.
Continuity mistake: In the sea hut scene, when Dudley steals the cake, there's a shot of him eating it and his pyjama shirt is pulled up, then it cuts to Hagrid, and he says 'He'll be under the finest headmaster...' and it cuts back to Dudley, but his pyjama shirt is back down over his pyjama pants.
Continuity mistake: In the scene straight after the Quidditch match where Harry, Hermione and Ron are telling Hagrid about their suspicions of Snape, you will see all three are wearing scarves tied in different ways. When there's a change of shots, the creases in the scarves change. This happens mostly with Hermione's.
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.
Revealing mistake: Harry, Ron and Hermione are at Hagrid's hut asking him about the hooded man and the dragon egg. The background behind Hagrid, which is the hut, is really behind him, but when it cuts to Harry, Ron and Hermione it is very apparent that the kids were edited in with their background, which is the school.
Continuity mistake: Just as Harry runs towards the wall to reach Platform 9 3/4, it is obvious that the owl in the cage is now a stuffed one.
Continuity mistake: In the scene where Harry, Ron, and Hermione are walking towards Hagrid after their exams, Hermione is carrying a brown satchel in her right hand. Then Harry says, "Why didn't I see it before!" and they run. While they are running she does not have the satchel in either hand, her empty left hand is visible in a few frames as she swings her hands at the side of her body. Then when they get to Hagrid she has the satchel in her left hand.
Continuity mistake: While the first years are waiting at the top of the stairs just before being lead into the Great Hall for the first time, Ron's robes switch from being fully opened, to closed, then back again.
Continuity mistake: After Harry thanks Hagrid for his birthday cake, Hagrid sits on the couch. In the next two shots facing Hagrid as he pulls out his 'umbrella', Hagrid's fry pan suddenly lies on the couch, which is visible to his right. The way he pulls out the 'umbrella' and the position of the fry pan differ within the two shots. Moreover, the fry pan is gone after these two shots.
Continuity mistake: The knotted tie string on Harry's Quidditch robe changes from extremely loose, to snug, to loose and so on, even in consecutive shots, from the time Harry speaks with Wood before they fly onto the pitch and throughout the match. This occurs with other members of both teams as well.
Continuity mistake: When Hermione is saying to Harry and Ron "You won't make a fool of yourself, it's in your blood," Harry and Ron have several books with them, but straight after, right before the staircase moves, they have no books in their hands.
Continuity mistake: In the scene where Professor Quirrell removes his headwrap to reveal the concealed Voldemort, Quirrell is obviously bald. However, if you carefully watch the scene with the troll, Quirell's sideburns are present after he shudders as the troll exhales, as they are in a couple of other shots.
Continuity mistake: When Harry raises his hand with the captured Snitch, his back faces the Slytherin and Hufflepuff towers which are opposite each other, with the hoops between them. However, in the next shot he is now facing that side of the pitch, as other Quidditch players appear from that side.