During a conversation in which Gus is lecturing Woodrow on his previous treatment of Maggie, both Gus and Woodrow state that Maggie died in Lonesome Dove. However, in the prequel, "Comanche Moon", Maggie is shown dying in Austin, Texas, before Gus and Woodrow decide to move to Lonesome Dove.
When Gus is dying in the doctor's office after Call comes to see him, there are crickets chirping at times. However, it is winter time and snowing outside. There would not likely be crickets chirping in winter Montana.
When Gus goes to shoot up the Indian encampment with July Johnson, count the number of Indians who "go down" from being hit versus the number of shots fired. One more Indian dies than the number of shots fired.