As the two boys drive away from the boarding house, the lady goes up to the steps, which are in the shade. The next shot, shows the boys farther away from the house which has the steps in full sunshine.
While not technically an error, the use of four separate levers and traps for the gallows was for dramatic effect and would not have been done if this was real. As seen in the quadruple executions of the Lincoln conspirators in 1865, the easiest way was to use one lever and one wide trap.
After Cahill is onto his sons' involvement in the bank robbery, we see him and Lightfoot watching the boys fishing. Later, after the boys have traveled awhile in the buckboard, we see the two men watching the boys again. The medium shot of Wayne and Brand reveals that they are sitting on their horses in the very place from which they had been watching the boys fishing.