Continuity mistake: When Bella goes to the Cullens' house for her birthday party, Alice takes the new camera that Charlie got Bella and takes a picture of Bella and Edward. But when Bella prints out the picture and puts it in her scrapbook, it looks nothing like the picture Alice took. Given we see the entirety of her visit from start to finish, there was no other opportunity for a different photo to be taken.
Continuity mistake: Bella just went back to the meadow and is talking to Laurent. Throughout the whole scene in the meadow, her hair changes on her right ear. It goes from behind the ear, to fully covering the ear, to half covering the ear, and back and forth between those. And in the beginning, her hair is near perfect, but by the end, her hair is more wispy and not-so-matted to her head.
Continuity mistake: In the scene where Carlisle is treating Bella's wound there is a shot of the bowl where he is putting the glass from the wound. You see that there is one piece of gauze. He then starts stitching up the wound and when he lights the contents of the bowl, you see lots more gauze that he hasn't used in there.
Continuity mistake: In the Volturi Chamber, when Aro moves forward to kill Bella after he says it's a sadness because Edward does not intend to give her immortality, you can see Alice's hair and part of her face along the right side of the screen. Actually, at this point Alice is standing to the left and behind Bella, being held by the neck by Dementri.
Continuity mistake: In the dirt bike scene, Bella takes off and starts to lose control. The camera pans so it is showing her hands on the bike, losing control, and she doesn't have a very stable control on the direction the bike is going. It looks as if she is going toward the right, into a small ditch, about to crash. The camera then shows her face, and she is still in the middle of the road, not close to crashing yet, with decent control of the bike.
Revealing mistake: When Bella and Edward return from Italy, Edward 'disappears' when Charlie enters the room. You can see behind the door when he is in the room that there is nowhere/not enough space for Edward to hide, but when Charlie leaves the room, as the door shuts, Edward steps out from behind it.