Sean Penn directs Dennis Hopper in this movie without Penn acting in it at all. Dennis had directed Sean in Colors (1988) which Hopper did not act in all.
After the chase scene near the end of the film between Joe in the police car and Frank in the Buick, Joe turns off the red police lights, then the car's headlamps and steps out of the car and stands next to it. In the final scene we see Joe still standing next to the police car but the car's headlamps have mysteriously come on again.
During the "mirror" scene, the "from behind" shot and the "mirror" shot don't match. (This actually makes sense, given that the camera would have been visible from the angle the scene is presented.)