The "gold" bars being loaded into the train are obviously fake slugs of undetermined material. You can clearly see the brush strokes of the gold paint that was used to create the illusion of a gold bar.
As Nobody and Jack face off in the New Orleans street - a window air-conditioner (draped with canvas) and what appears to be an electric window fan can be seen on the side of the "Hotel" in the background.
As Nobody is standing in the water waiting on a fish to come up and take the bait, there is a cricket on the water but it makes the sound of a fly of some sort. A cricket doesn't make a buzzing noise, not even in the water.
In Jack's first scene, he is told that his ship "sails the 21st." The telegram he receives in the saloon reminds him that the ship sails in 16 days, which would make the current date the 5th of the month. In the subsequent cemetery scene, however, Jack gives the date as June 3, 1899.
When Jack Beauregard and Nobody have their first street showdown, the sunlight and shadows clearly indicate late afternoon. Then Jack looks out in the distance and sees the Wild Bunch riders, who then ride into town. Now suddenly the sun and shadows indicate late morning or mid-day.
On the Street of Pleasure, Nobody meets a rude stilt walker and cuts him down to size. The sunlight first appears behind the stilt walker, then to his right, then his left, then in front, and then from behind once more, all in less than sixty seconds.
Opening Scene. In the barbershop when the would-be bushwacker has lifted the shaving cream brush out of the shaving cream cup and rotates it to stuff it into the barber's mouth, it is clean and dry. As he then lifts it to the barber's mouth and actually stuffs it in there is shaving cream all over it and subsequently, all over the barber's mouth once it's inserted.
The photographer is shown using flash powder to illuminate the gunfight; since the shootout takes place in bright daylight, this is unnecessary, and wouldn't give enough light to be useful at the distance shown anyway.