Continuity mistake: The angle from which Scorpio shoots the girl in the swimming pool does not match the way she is hit in the shoulder. (00:01:40)
Continuity mistake: When Scorpio shoots the girl in the pool a red towel is spread over the backrest of the right deckchair. When the police officers are dealing with the girl's body the towel is crumpled in the seat of the chair, but when Harry looks at the pool from the other building the towel is again spread out as before. (00:02:00 - 00:04:30)
Continuity mistake: When Scorpio shoots the girl in the pool she is swimming in the middle, but in all later shots the water is only bloody at the steps where she was pulled out. (00:02:05)
Continuity mistake: In the early scene when Harry walks across the street to confront the wounded robber on the street ("Did he fire six shots or only five?"), the shotgun used by the robber is lying several feet from the robber's hand, but when he reaches the robber, the camera shot shows the shotgun just a few inches from the robber's left hand. (00:13:00)
Continuity mistake: When the jumper jumps over to the cabin in which Harry was lifted up, Harry knocks him out, securing the man by holding his arm over the top railing. When they arrive on the ground he is holding the man's arm over the lower railing, which raises the question how and especially why he moved the unconscious man down while he was hanging outside the cabin. (00:31:10)
Continuity mistake: When Harry asks Bressler for some scotch tape to fix the knife to his leg the tape holder turns around before Bressler picks it up. (00:44:50)
Continuity mistake: When Harry is standing in front of the cross the light comes from behind. When the camera angle changes as Scorpio walks up to him the light comes from the side. (00:56:15)
Continuity mistake: During the scene on the football pitch where Callahan is torturing Scorpio to force him to say where the kidnapped girl is, there are several shots as if from Callahan's viewpoint. As the camera switches between the two characters, the white painted lines on the pitch mysteriously move or even vanish underneath Scorpio. (01:06:50)
Continuity mistake: When Scorpio leaves the hospital after treatment of his face injuries he has a band aid over his nose and a rectangular gash on the forehead. When he kidnaps the bus the tape is higher up his nose, and after he hits the little boy it is shorter. The gash on the forehead turns triangular at the end before they crash into the gravel pit area. (01:30:00)
Continuity mistake: In the scene where Scorpio takes the fishing boy hostage the shrubs behind Scorpio keep changing. Actually, there shouldn't be any as he is standing right at the edge of the lake. (01:34:45)
Continuity mistake: When Scorpio falls into the lake the ripples he creates disappear within seconds, but when the camera angle changes the ripples are back and there a lots of leaves floating on the water which weren't there before. (01:36:20)
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.
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.
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.
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.
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.