When Blackie puts the live round in the stage gun, it's a Colt single action. After Frankie shoots Johnny, the pistol is now a Colt double-action revolver that wasn't even introduced until years after the movie was supposed to be set.
While singing the song "Hard Luck", Johnny walks passed the large front windows of a general store. Several modern carbon-steel shovels are seen inside the window display, each prominently labeled with a large modern printed glued label by their manufacturer. This rather blatantly shows they were simply purchased from a store, and not true to the design and style of shovels made during the setting of the film.
Johnny's talisman could not stop a bullet fired at point-blank range.
When Johnny pulls down Cully's beard during the Mardi Gras party, Cully says something as he fixes his beard, but his lips never move.