Revealing mistake: When Bella agrees to drink human blood to save herself and her baby, she says: "I'll try anything." If you look closely at her mouth when she speaks, you can see that even prior to her first sip of blood, there is a red substance between her teeth from a previous take.
Continuity mistake: Charlie hands Bella off to Edward at the altar, she is carrying her bouquet, the next shot is from behind them, Bella, Edward, and the pastor in the shot, she still has her flowers. In the next frame, from the front, it's Bella, Edward and the pastor's back, flowers are gone, and they never show up again. She didn't hand them off, there was no one to hand them too.
Continuity mistake: When Jacob runs outside the Cullen's house to stop the wolves from fighting them. He leaps off the steps and runs towards them. Before the shot changes he is right beside Edward and another wolf, however, in the close up shot of him saying "Stop!" suddenly he's in front of Edward.
Continuity mistake: When Alice is putting in Bella's hair clip before the wedding, she puts it in once, and the shot changes angle and you see her put it in again. It's because the girls change places too, first Alice is on the left and Rosalie is putting the clip in her hair, it pans out and back at them and then they are on opposite sides and Alice is putting the clip in.
Continuity mistake: When Jacob and his pack meet with Sam's pack, Seth and Leah are standing behind Jacob the entire time. At the end of the scene, when Sam's pack leaves, we cut to a shot of Jacob from the same angle as the previous shots where Seth and Leah were, and they are gone, with no indication that they left.
Continuity mistake: As Edward and Jacob are working to revive Bella just after she gives birth, Bella's eyes keep changing from being open and unblinking to tightly closed. The "making of" documentary on the "DVD Extra", showed a life-size dummy of an emaciated Bella with closed eyes that was used in more physical scenes. You can tell it's the dummy whenever Bella is shown with her eyes shut.
Continuity mistake: When Bella is first seen pregnant, she is wearing grey pants, a blue shirt and navy jacket, Edward is wearing a brown shirt, and Carlisle is wearing a blue mock turtleneck. On what is supposed to be the next day (and after having seen them wearing different clothing), all three are wearing the same clothes again while in the library/makeshift hospital room. They are discussing a cracked rib that was causing Bella pain the day before. These scenes were obviously meant to be back-to-back, but were rearranged in editing.