In a scene depicting a detective scoping out D.C., the completed WW 2 Memorial is clearly visible in the background. The memorial construction was started in September 2001, the sniper killings occurred in 2002.
Malvo is repeatedly shown firing through the tail light assembly, which is removed and replaced as necessary. Malvo actually fired through a hole cut into the car's body above the license plate.
Whenever Lee Boyd Malvo is aiming they show a sniper crosshair, even though the rifle doesn't have a scope.