At 1:32:40 when Bass in in the bedroom during a shootout a pot belly stove with a flue pipe can be seen. The flue has duct tape wrapped around it. Duct tape did exist at the time and duct tape would burn when wrapped around a hot flue.
In one of the last town scenes a modern storefront is shown with large tempered glass windows that have extruded aluminum frames. Only small glass panes were available in 1875 which would have been glazed in wooden batten frames.
Almost all hats used in the movie are modern design. In several shots you can see the tag inside of Ron Perlman's hat.
In the penultimate scene of the movie where Bass is riding into Ft. Smith, AR, a modern flagpole is seen with the U.S. and Alabama (state) flags.
The Blacksmith depicted in the movie has a sign that reads Vulcan Forge AFC. Vulcan Forge is a modern day not for profit teaching organization who belong to the Alabama Forge Council (AFC) which was formed in 1985 also as a not for profit organization. Possibly placed as a trade off for the use of blacksmith equipment in the movie.
When Charlie is taken back to a room to sober up, an electric cable can be seen running up the stairs powering the light.
As Bass carries Ron Perlman out of the mud pool and hand can be seen feeding the horse to keep it standing still in the mud.
At the 2:42 mark in the beginning, there are two porta-potties at the edge of the field in the distance.
When Bass is removing the wanted man from his horse in the mud, the legs of the crew member holding the horse steady can be seen through the horses legs.