In some of the shots where there are horses pulling a wagon, it is clear that the trails they are on were made by modern vehicles. Clearly if they were wagon trails, the wheel lanes would be smaller and the center of the trail would be torn and scattered from horse's hooves.
The guitarist in the bordello is asked to sing "I'm a good ole rebel". The song was only copyrighted in 1915. Although it is possible that it was known in the period portrayed (early 1870s), it is unlikely that it would be so widely known that the performer would have been able to play it from memory. If so, it would beg the question why the author had not copyrighted.
When the wagon trains and horses are shown riding down the street in Northfield, Minnesota before the bank robbery, the clacking of horses' hooves can be heard (the sound they make when walking on pavement or concrete) even though they're walking on dirt.
During the Northfield bank robbery, when the bank customer is shot running out the door to warn the town, he is shot in the back and blood splatters on the glass of the door. In subsequent shots of the door the blood splatter disappears.
When Bob Younger is thrown from his horse in Northfield, Minnesota, and in the next two shots of him as he struggles to his feet under fire, he is holding a revolver in each hand. When the shot next cuts to him, he is frantically working and firing a Winchester 1866 (Yellow-boy) rifle. Presumably this came from the boot in his horse's saddle, but he has had barely a second in which to retrieve it.
During the Northfield bank robbery, a bank customer is shot running out the door to warn the town, and falls in the street with his suit jacket bunched up towards his head. In subsequent shots, his suit jacket is all the way down.
Cole Younger (David Carradine) is shot in the right leg and arm while riding his horse in Northfield, As each squib is activated, the button in his right hand used to fire the squib is visible. The wire leading from the button up his sleeve can be seen when he is shot in the arm along with gaffer's tape on his shirt to hold something in place.
When the brothers are making their escape after Northfield Bank robbery, Cole Younger (David Carradine) is shot in the leg. You can see the detonator for the detonator in his right hand and the wire going down his sleeve (1:39:40).
The revolver Frank James threatens Rixley with and carries throughout the movie is a Smith-Wesson Schofield. In reality, Jesse carried a pair of Schofields while Frank preferred the heavier Remington New Army (the type of pistol shown carried by Cole Younger). After leaving prison, Frank James did a print testimonial for Remington referring to the Remington as 'the finest hand gun I ever carried'.
During the Northfield raid, when the steam engine came up the street, the tracks from the flat wheels and the heavy weight could be clearly seen in the dirt of the street, but on the next scene as the riders were coming up the street, there were no tracks visible.
During the knife fight, the position of Sam Starr's holster and knife switches from left to right during a brief shot while Cole's knife is switched to his left hand indicating that the shot was mirrored.
The goof item below may give away important plot points.
In the Northfield raid Clell Miller fires his pistol at a bank employee, and hits him right in the forehead, and the man is shown to have a wound there, just as it should have, but also, the viewer sees a splash of blood, roughly six inches, diametrically. Highly unlikely, with most any handgun that blood would be blasted out of the man's head like that, although the bullet might make it all the way through his head. Now, if this had been a frangible bullet, this could happen, but dum-dum bullets hadn't been invented then anyway.