Factual error: While arguable that Glass could have survived the numerous attacks upon his person (a momma grizzly, cold, hunger, starvation, etc.), his lengthy immersion in an icy river and subsequent grounding and re-warming boggles belief. While hypothermia is considered to be the cause of death in such instances, the fact is that within minutes in extremely cold water, a person becomes unable to perform the simplest of motions as the extremities shut down...and they drown. In addition, Glass hauls himself from the river and shortly thereafter is seen in his warm, dry outfit, heading out for new forms of physical and emotional abuse.
Factual error: Fitzgerald tells a story about his father setting out with a party of Texas Rangers across the Llano Estacado. There are two problems with this: first, the Texas Rangers were first formed in 1823, the same year the movie takes place, and consisted of around 10 men who were not yet known by that title. Second, the Llano Estacado is in the Texas panhandle, an area almost completely unexplored by Anglos at the time (and nowhere near the area where any rangers lived).
Factual error: When the Indian band finds the body of Glass's son, one of them clears the snow off his face and touches the skin. The skin is easily indented by the touch, like the skin of a living person, This scene is in the winter in the high frozen mountains. The boy had been dead for at least a day, and the body should have been frozen rock solid.
Factual error: Stars in the sky are misplaced. The constellation Orion is on the wrong side of Pleiades and Taurus. In the movie Orion is placed above and to the right of the other two. In actuality, it is below and to the left of them in the sky. It appears that Orion was pasted into the frame, since the other constellations appear in their correct locations with respect to one another and the moon. (01:16:10)