When Ellen has her gun first fight, it happens at 7 p.m. Given the period the movie was set (1800s), it is way too bright for 7 p.m. There was no daylight savings time and no matter what time of year it was, it was too bright for 7 p.m.
About 25 minutes into the film Sharon Stone's character is at the bar drinking and "Kelly" challenges her to a shoot out. There is a Standing Liberty Quarter visible on the bar, presumably in payment for the drink. The film takes place in the 1870's, but that type of quarter was not made until 1916.
During the sign-up for the contest, Ace tells the barkeep "Put an ace up there, everyone will know what it means." The barkeep actually draws a spade, which is one of four units in a standard deck, that could represent any card from ace to King.
When The Kid draws against Scars, the closeup shot appears to show him shooting left handed, but the film is actually reversed, indicated by his revolver's ejector rod being on the wrong side of the gun's barrel. After Scars fires, the distant shot of The Kid shows him holding his revolver in his right hand.
Ratsy leads Cort to the street to fight Spotted Horse, starts with wide right pan and Ratsy is to Corts' left ending that pan. Next shot is of their feet and pan up to see Ratsy is now on Corts' right.
(at around 22 mins) Eugene Dred is offering the bar girl a piece of jewelry in a "business discussion." On closer inspection it's a ring on a chain, the same ring he is seen taking off the late Ace Hanlon's left pinky and stringing on a chain less than four minute later at 46:02.
This is similar to, but not exactly the same as other goofs. Immediately before the gunfight between Herod and Ellen, Herod's shadow is at his right, behind him. Immediately after the fight, Herod looks down at his shadow (and the hole) directly in front of him.
During the fight between Cort and Herod, they are both supposed to be using their left hands. Between explosions, Herod is again seen with his gun in his left hand. However, he re-holsters his right hand gun before the fight with Ellen.
(at around 1h 35 mins) When Herod gets up just after the explosions, the camera pans up. In the background on the right side is a shadowy figure (perhaps a pyrotechnician) wearing a thick flak-vest and holding a radio control.
The entire idea the movie is based on, with two opposing gun fighters meeting face to face in the middle of main street to quick draw, is actually extremely rare to find actual examples of in the American Wild West. With the exception of the famous "Wild Bill" Hickok - Davis Tutt shootout on July 21st 1865, finding documented occurrences of this is extremely difficult. In reality you would try to shoot your adversary in the back, or it would be some other ambush scenario.
A bullet fired from a gun held by an adult cannot carry enough energy to pick up an adult and throw him back several feet. To do that, the shooter would also be thrown back several feet, by Newton's third law of motion; or the gun would be too heavy to carry.
In the final gunfight, Ellen's bullet is shown going towards Herod's eye. This would tend to knock him backwards from the top. A brief shot, confirms this, showing Herod's legs pivoting backwards from the top. But the next shot shows Herod flipping heels over head and landing face down. This would only be possible if he had been hit near his feet, and from behind.
About sixty-three minutes into the picture, when Herod has the duel with Sargent Cantrell, we're shown Herod firing his weapon at Cantrell's head and when the bullet hits Cantrell, suddenly there's a huge hole that appears all the way through the Sargent's head, and it is about the size of an egg: perhaps four inches or so, diametrically. Such a large wound would be impossible with such a small caliber weapon as Herod was characterized as having used in the scene.
Herod challenges Cantrel to a fight. When this happens, there is a heavy rainstorm occurring. However, when they fight, the ground is dry. Several times in the movie, scenes with rainfall are followed by scenes (according to the story happening right after the storm) where the ground is completely dry. There were no gun fights on this day due to the rain. The fight between Herod and Cantrel takes place the following day.
Instructions are given for the fights to commence at the first chime of the hour on the clock. So much attention is given to the click before the strike, that the clock is never actually heard chiming.
When Ellen in supposed to be on horseback at the beginning of the movie, she is standing still looking upon the town. When the gang of horsemen run by her, the camera's frame is too big because you can see she is pretending to hold on to reins. In actuality, she is not sitting on a horse and she is holding up nothing.
When Scars is dragged away during the sudden downpour, his legs and feet drag the wet dirt. But neither he nor the bearers leave any marks in the mud, revealing it to be cemented soil to make film production easier.
The goof items below may give away important plot points.
When Ellen tosses the Marshal's badge at Herod, it lands in front of him near his feet. When she shoots him the second time, the force of the bullet throws Herod several feet backwards. However, when Ellen picks up the badge it is still between Herod's feet.
At the end when Ellen shoots Herod in the center of his chest, the sunlight shows through Herod's shadow. However, the sun is high in the sky and cannot possibly show through a foot long hole through Herod's chest. The sun would have to have been setting behind him.
When Ellen appears for the last fight scene, she flicks (from her chest without a back swing) her father's Marshal badge at Herod's feet. A couple of scenes later you can see that the distance between the two of them is too far for her to have reached his feet. After Herod is killed, Ellen retrieves the badge from the ground and tosses (with a back swing from the hips) it at Cort just reaching him. The distance between the two is shorter.
The idea that the kid would have died slowly from a stomach wound is not necessarily correct, however there are several arteries in the abdomen that could have been severed which would have caused him to bleed out very quickly as shown.