Revealing mistake: When Alejandra comes to John's room the first time you see distinct t-shirt marks on John's upper body, his arms as tanned as his neck and face. This in spite of the fact that he wears long-sleeved shirts throughout the movie (except, of course, during his nightly encounters with Alejandra). (00:49:30)
Revealing mistake: Jack Elam gets bothered by a fly crawling over his face (nice stunt, actually). When the fly moves over the bench, just before he catches it with the muzzle of his gun, look at the hind legs. You'll notice that the fly is dead. (00:07:50)
Revealing mistake: In the closing scene, you can see the modern curb & sidewalk have been covered with dirt.
Revealing mistake: When Blondie is to be hanged by Tuco, and saved by an extremely lucky hit by a Union mortar, you can see that the wall that will be hit by the grenade has been blown to pieces at least once before; the bricks have no mortar between them, and there is a large circular scorched area on the brick wall. (00:47:10)
Revealing mistake: When Nullah and his mother are being pulled out of the tank, the stitching on the dummy used for the mother is visible.
Revealing mistake: When Howard and Curtin chase Dobbs, they pass a dead donkey which is obviously breathing, and the rope used to tie its legs together is visible too.
Revealing mistake: Early in the movie, there is a visible circular, plastic looking effect on Gideon's arm as he removes the bullet with the knife. This is more noticeable around his shoulder when he cauterizes his wound with the hot knife.
Revealing mistake: When The Indians launch their final attack (towards the end of the film), on a old outpost held by a small group of soldiers, Ward Bond is hit by a arrow which enters the right side of his chest. In the next shot you see Gregory Peck removing the arrow from the left side of Ward Bond's chest.
Revealing mistake: When Captain Quincy Wyatt interrogates a captive Seminole Native American by threatening him with a poisonous snake, the native is safe because there is a protective piece of glass between him and the snake. The snake's reflection is quite obvious on the glass panel. (01:14:40)
Revealing mistake: Towards the end of the film when the old guy (Ed Begley Sr) is shooting a rifle at Clint Eastwood, the thumb of his left hand is on the top of the barrel, preventing him from seeing the front sight.
Revealing mistake: When Chris is being hanged you can see the back of his jacket being pulled up from the harness.
Revealing mistake: The baby Robert William Pedro is actually a girl. You can see it in the scene where the baby is greased.
Revealing mistake: When Fogg pulls the knife out of Passportout's buttock you can see that the knife is attached to something under his pants.
Revealing mistake: A 'bald wig' is visible on Harry Morgan when he first arrives in Purgatory.
Revealing mistake: When Alferd runs from the saloon with a lynch mob behind him, we can see a line in the dirt ahead of him that serves as a marker for where he is supposed to run.
Revealing mistake: When Clint Eastwood is shooting his 1896 Mauser Machine pistol, with the long barrel, even though he fires numerous shots the hammer remains fully cocked and does not move.
Revealing mistake: Following their escape from the comancheros, the two wheeled wagon driven by the old woman had a set of reins coming through the slats beneath the seat indicating that the old lady held fake reins and the wagon/horses were controlled by someone inside. Later, when they left the small town and headed to her son's ranch, a little window had been cut out of the slats where the control reins came through, no doubt to improve visibility and horse control.
Revealing mistake: When Chino does some rodeo riding when he breaks in one of his horses we get a good view of the stuntman's face. (00:33:50)
Revealing mistake: When Eastwood shoots off a portion of the bad guys ear, the resulting portion of the ear is very fake looking.
Revealing mistake: Early in the movie when the maid is opening the drapes to let in the morning sun, the sun lights the room a half a second before the drapes are actually opened.