9 minutes into the film, as the elevator door closes, the last Passenger puts his arm in the way to stop it, but it is not visible in the mirror. This reveals that, to avoid revealing the camera in the mirror, they superimposed video into the scene.
The goof items below may give away important plot points.
When the salesman dies in the elevator, Detective Bowden gets on his radio and says "we have a possible 187." 187 is the California penal code section for homicide. This movie is set in Philadelphia and there is no penal code section 187 in Pennsylvania.
In the scene where Tony has a flashback to when he crashed the car into Detective Bowman's wife and kid, you can see that when the car spins into the ditch, the license plate is backwards, meaning that the clip was reversed.