When Rex rescues the "Cattle Barons Daughter" from being dragged around by her horse, his gun belt starts falling off as he is running over to her. You can see it down by his knees.
When Rex O'Herlihan walks into town before the final showdown, you see Colonel Ticonderoga move his coat back behind his gun, making sure he has access to it, the next shot the coat is covering the gun.
After the big shoot out at the end of the film, in an overhead shot, the Railroad Colonel briefly disappears.
When we first see Colonel Ticonderoga, he is holding his pocket watch, then puts it in his vest pocket. But when he turns around, he is holding the pocket watch again.
In the final shoot out scene, there is a three shot and the Colonel is seeing pulling his coat back behind his gun. In the next shot seen from being Rex and the sheepherders, the Colonel is clearly seen with his coat still covering his gun.
When Peter (the town drunk) shoots the hat off of Rex's head by mistake (aiming at the cigarette Rex has in his mouth), you can see the string that pulls it off.
Early in the film, when Rex is singing, you can hear the noticeable difference between his singing voice and his speaking voice. Obviously, Tom Berenger's singing voice was provided by someone else.
When the train is stopped, and Rex shoots back at the three men with the rifles, you can see the string that pulls off Jim's hat in the background.
You can also see the person behind him pulling it off.
During his sidekick training, G.W. Bailey (Who was a classmate of Janis Joplin in Port Arthur, Texas) sings while watching his left hand not changing chords.