Other mistake: When Harry, Ron and Hermione rush to Hagrid after the end of the year exams, and Harry is saying something that ends with, "Why didn't I see it before?", Hermione is mouthing part of that line. [Emma Watson did admit she learned Dan and Rupert's lines as well as her own and found herself accidentally mouthing them.] (01:50:40)
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)
Other 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. (01:10:43)
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 were looking up towards him. (01:20:40)
Other mistake: When Neville flies away but falls off it is obviously animated. When he falls behind the crowd of students they are covering up Matthew Lewis, the actor who plays Neville, but the place where Lewis and the animated Neville fell are different. (00:54:45)
Other 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)
Other mistake: In the scene where Ron and Harry are on the train to Hogwarts, the window to the compartment is open. We know this because the chocolate frog jumps out the window and is swept away by a pretty strong wind. Since the window is indeed open, shouldn't there be a visible breeze thoughout the compartment? (00:35:50)
Other mistake: In the scene near the end of the movie where Harry boards Hogwarts Express to go home, Hagrid stands on the platform and waves goodbye. He is wearing a coat and looks huge, as usual, but you can see when he waves, that his hand seems rather small compared to the rest of his huge frame. (02:23:15)
Other mistake: While Harry and his classmates are on their way up to Gryffindor Tower right after the Sorting, Neville reacts with astonishment to a portrait moving. However, we learn in later films that Neville was raised in a magical household (by his grandmother), so he should have been completely accustomed to seeing pictures move.
Other mistake: Near the beginning, when the Dursleys' house begins shaking, none of the objects fall nor do the curtains shake.
Other mistake: Percy Weasley is obviously talking to the Gryffindor first years when they first enter the Gryffindor common room, but there are too many first years. There are 5 boys and 5 girls in each dormitory, so 10 first years in each house, but in the shot there are obviously almost 20 students there, not including Percy. (00:47:30)