Other mistake: In the beginning credits of the movie, we see Harry Callahan approaching the swimming pool where the girl was shot. Next we can see some blood in the water of the pool. By the time Callahan gets there, the blood should have diffused already.
Revealing mistake: When Harry is drinking his cup of coffee with Lt. Bressler at the police station, look at the cup Harry is drinking from and you can see that is empty.
Continuity mistake: When Scorpio tells the children in the school bus that they are going to the ice-cream factory, there is a boy wearing a red T-shirt and he is sitting next to the aisle. When the camera changes to behind Scorpio in the next shot, the boy is now sitting next to the window.
Audio problem: When Harry shoots Scorpio in the leg on the football field, as he walks towards him, Scorpio shouts out "Don't do anything more! No! No!" If you look at Scorpio's face, however, you can see his mouth is not moving in sync with what he is saying.
Continuity mistake: When the boy in the school bus says he wants to go home to see his mother, he raises his hand. In the next shot when Scorpio goes up to him, his hand is seen lowered, and when Scorpio smacks the boy, his hand is raised again.
Revealing mistake: When Scorpio robs the liquor store, the bottle that he was going to purchase is used to hit the clerk over the head, but you can see it is empty.
Revealing mistake: In the sidewalk scene, after one of the robbers leaves the bank and dives into the car, and it drives towards Harry, you can see a wire rope that appears to have been used to either stop the car from going too far or has been used to help roll the car. (00:02:07)
Continuity mistake: The scenes at the quarry were obviously filmed on different days. When the kid is being held hostage, the sky in the background is different from what it was minutes ago.
Other mistake: Due to the way the opening titles were created, likely with an optical printer on plastic or glass, dust or other particles are very visible when they fade into the shot.
Continuity mistake: Before Scorpio pulls out of the knife that is stuck in his leg the top of his trousers is completely clean, then dirty.
Continuity mistake: When Harry confronts Scorpio at the lake at the end of the film, Scorpio picks up his gun and stands up after Harry says "Well, do you, punk?" When Harry fires the bullet that kills him, Scorpio is now crouching down.
Continuity mistake: In one scene, Harry's bulldog is an un-neutered male. The next scene in which the dog appears, it is a female.
Revealing mistake: When Scorpio slaps the boy in the school bus, you can see that Scorpio's hand actually misses the boy's face.
Other mistake: When Inspectors Calahan and Gonzalez, the chief of police and Lieutenant Bressler are planning the nighttime sting, Gonzalez asks how the rest of the group knows Scorpio will enter the one particular rooftop. The chief of police advises Gonzalez that criminals have behavior patterns. In an earlier point in the film, Gonzalez states he has a bachelor's degree in Sociology. Although profiling and criminology often are studied under psychology, he should at least know how people behave and interact as sociology is the study of patterns of social relationships and people.
Answer: He certainly knew at the start, which is why he felt confident in pointing the gun at the (disarmed) robber and pulling the trigger. Harry knew he was bluffing, but passed it off so well the robber wasn't sure. At the end of the film, again, he knew he had one bullet left and wanted to bait Scorpio into attacking him, giving him an excuse to fire.