Other mistake: When James Marsden and Joshua Jackson are fighting outside the prison, you see a close up of Joshua hit with a flower-pot that then cracks. A wide shot is then shown and not only is the flower-pot now intact, it wobbles when it hits the ground and when James moves it suggesting it's a fake rubber pot.
Plot hole: Towards the end we see Jones ring Derrick to warn him that Travis has a gun - why would she do this if it's all planned? why risk him picking the phone up? he would have escaped before Travis had time to stop him and get him back inside to finish the set up. This scene is purely for the audience to be more taken in and shocked at the actually twist.