Continuity mistake: When Malfoy and his friends bother Ron and Hermione, Harry shows up and messes with the three boys. He grabs hold of Malfoy's feet, lifts them in the air and pulls him across the snow. In the first close-up of the soles of Malfoy's boots, not a speck of snow is on them, though he was just walking in it! Then in the next close-up, his feet still in the air, the soles are now covered in snow.
Visible crew/equipment: At dinner, just after Harry takes the plate from Petunia and as Aunt Marge says, "A bit more," referring to the brandy Vernon pours, in the left glass door of the hutch behind Vernon, there is a nice clear reflection of a crew member lowering his white sleeved arm off screen.
Deliberate mistake: When Harry drops the Monster Book of Monsters, in the first shot facing Harry's feet as the book chomps at him, he runs backwards a few steps and he stands at the foot of the bed. However, in the shot facing the fireplace, there is actually plenty of floorspace now, much more than other shots, between the rug and bed for Harry to continue to run backwards.
Trivia: In order to acquaint himself with his three lead actors, director Alfonso Cuaron had each of them write an essay about their characters, from a first-person point of view. Emma Watson, in true Hermione fashion, went a little overboard and wrote an 16-page essay, Daniel Radcliffe wrote a simple one page paper, and Rupert Grint never even turned his in, as he said that is what Ron would have done.