Revealing mistake: At the end of the film Vanessa gets crazy because Bob kills her grandma, and she starts beating him up in the caravan, hitting him and throwing various stuff at him. Throughout the scene, Reese Witherspoon's' stunt double is really obvious. Her hair looks completely different!!!
Continuity mistake: When Bob climbs out of the car, running away from Vanessa he runs in a straight path from the drivers seat, very far away from the van and is then shot by Vanessa. Moments later when Vanessa robs him, he is closer to the van and near the rear of it. He is in a different location and his body has also rotated.
Revealing mistake: When Bob and Vanessa are battling at the end of the film, he fires 22 shots before he dies. Besides the obvious fact that the gun doesn't hold that many bullets and wasn't reloaded, there was no sparks or smoke from the gun on almost all of the shots and although he seemed to be firing randomly around the room, there are no holes in the walls or even in window like there should have been, especially seeing as a couple of shots made it outside to the policemen.
Audio problem: When Bob runs from Vanessa, out of the car after being shot, Vanessa shoots her gun, and we see the bullet hit the floor, with the audio of gunfire to match it. Then audibly we hear Vanessa shot the gun two more times, however visibly three bullet-holes appear in Bob's brown coat, so they forgot the audio for one of the bullet-fire, also revealing that Bob was probably wearing squibs.
Other mistake: At the end when Vanessa is sitting in the deckchair outside her Grandmother's caravan, she asks the cops if they have a cigarette. The view cuts to the cops who smile at her and chuckle; when the view cuts back to her, she grins and you can see a faint plume of smoke rising from below the camera, indicating she already has a lit cigarette. She didn't light one at any point either.