Continuity mistake: When the Preacher is tossing dynamite into a barracks shack, a bad guy stumbles out and drops a rifle on the steps, where it lands leaning on the them. In the next view, it's disappeared, then it reappears several feet away, lying flat on the ground.
Other mistake: At the final showdown with Stockburn, the Preacher wields a cap-and-ball New Model Army Remington that requires a cylinder change to reload. Why does he have a gunbelt full of thumb-size.45 Colt cartridges? They'd never fit in the little sheriff's model, even if it had been converted to a cartridge revolver.
Factual error: On several occasions during the film, one hears the sound effects of cartridges being ejected from revolvers, then reloaded. The revolvers carried by every character in the film appear to be of the "cap and ball" variety, which do not take cartridges and take MUCH longer to reload. Can't be the sound of the cylinder being removed - the cylinder in the closest revolver is visible throughout the sequence. In addition, several of the revolvers are Colts, which require a wedge and then the barrel to be removed before the cylinder can be changed. Only the Remington (which Eastwood carries) could have the cylinder swapped quickly enough.
Continuity mistake: At the beginning, Hull watches as Eastwood beats up the guys in town. When Hull gets up on the wagon, his hands are fine, but in the next scene he has a bandage, I believe, on his left hand. Then the bandage is gone again and reappears soon after.
Visible crew/equipment: In the scene where the preacher has left camp prior to the showdown with the bad guys, a shot at camp shows the cabin in which those who took him in stay. From right to left at the top left of the screen looking at the front of the cabin to the left top you can see power lines. No power during this time.
Revealing mistake: After the cowboys have terrorized the village at the beginning, the girl finds her dead dog and starts crying, when she picks it up it's painfully obvious it's a stuffed toy.
Continuity mistake: Eastwood puts five or six bullets into the lead bad-guy at the climax of the big shoot out. As the bad-guy Marshall pitches forward, you can clearly see the bulging outline of the blood pack beneath his clothing.
Continuity mistake: Near the end of the movie, Eastwood is tossing sticks of dynamite into the "bad" miners' camp. He tosses one stick into a tent. After it blows up, you can still see the stick of dynamite on the ground at the entrance to the tent, with the fuse still burning.
Continuity mistake: When Preacher is about to kill Stockburn, as he walks toward Stockburn he reloads his revolver by replacing the cylinder. He briefly points the pistol at the camera, you should be able to see bullets in the chambers of the cylinder but they are empty. Again, just before Preacher actually shoots Stockburn, he points the revolver at the camera and you see empty chambers where you should see the unfired bullets.