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.
When Mick falls on the unicorn, there are three quick shots of him being impaled. The first and second shot show him straddling the unicorn with his leg straight. The third overhead shot shows his leg wrapped around the brass pole.
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.
The movie ends at the merry-go-round. As the credits start and the camera pulls out, the roller coaster and the merry-go-round are far away from each other. It is impossible that a person could fall from the roller coaster and crash through the roof of a construction that far away.
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.