While Harry is nailing up the targets, his shoulder holster has some sort of odd cross strap; as if someone in wardrobe just pieced it together. Moments later, while waiting for Horace to walk up on him, Harry's shoulder holster is the traditional Bianchi shoulder holster configuration - a single elastic strap around his right shoulder.
As Horace exits the car while visiting Harry, who is about to begin target shooting in the woods, one of the Panavision cameras is clearly visible, reflected in the car window, as well as Eastwood's son, who was visiting the set.
When Callahan is attacked by the four Mafia hit men, they start firing at a wooden box they suppose Harry is hiding in. One of them fakes shooting a pistol, mimicking the recoil, as it is clear that the gun doesn't fire at all.