Plot hole: It is highly unlikely that something as unusual and noticeable as a long severed ponytail would have been overlooked at the crime scene where Bob was shot when it was originally combed for evidence. Especially after Vanessa told the police he'd done it; they would have looked for it as evidence.
Continuity mistake: When Vanessa is sitting at a desk, opposite the two detectives (before she starts talking about what happened - it's chapter 8 on the DVD), her hands keep on changing position between shots. In one shot they're resting on the desk, then they jump to resting on her knee, then on the desk again, and so on throughout the scene. (00:52:25)
Visible crew/equipment: The red microphone is visible, hanging over Vanessa's head and bobbing into view when she says to Rhonda (when they are sitting on the jail beds) "They put you in there for that." It's visible again after Rhonda says that she loves girls, in the shot when Vanessa replies "You can keep 'em." Then when Mesquita comes over to start trouble with Vanessa and says "Bitch. I'm f*cking talkin' to you, man" to her, it's visible, hanging over her head and bobbing into view once again. It's very visible many times throughout the movie. Again, near the beginning of the movie, in the shot when the female police officer says to Vanessa "Social services are on their way," and Vanessa replies "Oh that's just great."
Continuity mistake: When Vanessa leaves Bob for dead, she has no blood on her face. She goes to a cafe and suddenly there is blood all over her hands and face; the amount on her face suggests it would have been visible to viewers when she left Bob. The light wasn't dark enough to hide so many stains.
Continuity mistake: When Rhonda and Vanessa chat, while they sit on the jail beds, Rhonda's arms/hands keep changing position between back and front shots. They could be resting in her lap, then up fiddling with her hair, then she could have her head resting on her hands etc. It randomly changes throughout the scene.