When the horse with Gable's son runs away. the child is originally on the right side of the horse. The succeeding shot shows him on the left side of the horse. Later shots show him back on the right side. The anomalous shot is shown again later with the baby now on the correct (right) side, indicating that the film had been flipped.
This movie took place around the 1828 time period and most of the firearms used were percussion rifles, but the percussion cap was not developed until the early 1830s. Hawken, the most renowned rifle coveted by hunters of the time, did not release a percussion rifle until around 1839.
It's quite obvious that the papoose on the runaway horse is a doll.
Early in the movie (at the 7:12 mark), when Kamiah is talking to Flint about trading horses for a wife, there is a motor vehicle in the lower left corner driving along a road in the distance, although this story took place long before the automobile was invented.