After the town has been painted red, a shot pans past the bell tower atop the church. The (lack of) application of paint on the bell tower edges shows it was painted with a spray gun. Also, many of the buildings have a very thin and even coating of red paint applied. Again, obviously done with a spray gun. This technology would not have been available at the time.
When the Stranger is first sipping his beer at the saloon, the bottle of whiskey is placed on the bar to the left of his glass of beer. When he reaches for his beer while saying the line "Faster than you'll ever live to be" to the one gunfighter, the bottle of whiskey "jumps" to the right of his glass of beer so he can pretend to draw his gun yet reach for the bottle of whiskey instead.
The morning after his first night in Lago the stranger steps out of the hotel and the mayor says "good morning." The shadows on the ground clearly show that it is late afternoon as Lago is located on the western shore of Mono Lake.
At the ambush, the stranger places the stick of dynamite just back from the edge of the slope. When it explodes, the explosion is a couple of feet down the slope from the edge, not where the dynamite was placed.
As The Stranger arrives in town the Saloon is clearly lit from the front (the word "Saloon" is shadowed on the window shade). After tying up his horse, the stranger walks to the saloon which is now in shadow.
As the Stranger rides through the town for the first time, Sarah Belding looking down from the second floor balcony is in sunlight when seen from the front, then in shadow when viewed from behind, then back in sunlight again when viewed from the front.
Regarding one of the three bad guys who confront the Stranger in the barber shop. The tall guy with the beard who is shot and crashes through the window into the street. As he attempts to support himself against the hitching post one can clearly see the shape of an elbow protector for his left arm under his shirt.
When the bad guys return to town for the final showdown, one of the bad guys on horseback knocks one of the townspeople over a table in the street. As the table is tipped over you can clearly see a moving blanket under the table cloth there to protect the stuntman.
When the town men are in the church debating their predicament, there is a plaque on the wall that quotes a scripture. Isaiah 53:3-4 is the scripture address written at the bottom of the plaque, but the scripture written out on the plaque is only Isaiah 53:3 (kjv); verse 4 is missing.
When Stacey, Cole and Dan come across three campers in the mountains, they have plenty of ammo despite their guns being given back to them empty when they left prison. However, it can be seen that their gun-belts still had ammunition in them.
When the next to last bad guy is hanged with a bull whip, as he is first hoisted he spins a little and you can clearly see that is is a rope around his neck (not a bull whip), and that it is tied in a knot with some other ropes going down the back of his shirt (obviously attached to a supporting harness).