When they first try the weapons, Paul says to Barbara that the shotgun chamber can hold only three shots, so she should make each shot count. When he kills the Outsider at the movie's end, Paul uses the same shotgun and fires off four shots in a row.
The Outsider slashes the back of Paul's flannel during the fight scene in the tunnels. His flannel and his flesh are torn and bloody. Moments later, Paul confronts the homeless lady with the tracking device in her cart, and when he turns away, the back of his flannel isn't torn at all.
Paul's hair is all messed up when he asks the guys in the station wagon where he is, but then his hair is combed in the next shot.
When Einstein approaches Barbara's computer, it's visibly in her apartment. However, the background switches to a black void when the scene cuts to Einstein picking up the pencil.