For the boys' meal at gunpoint with the farmer, the food is repeatedly described as being "sowbelly and turnip greens", which is a boiled dish, yet what they are eating is mashed potatoes and uncooked spinach leaves.
When Jake is describing what Simms is doing at the stagecoach, he says, "He's opening the door ... he's getting in ... he's closing the door," but when we see the stagecoach, it's driving away and Simms has only just managed to open the door.
When the boys cross the Missouri River from St. Joseph, one utters the words, "Say goodbye to the U.S.A." After January 29, 1861, what was across the river from St. Joe was the state of Kansas, so they never left the U.S.A. In fact, by the 1850s, to ride out of the U.S.A. they would have needed to have gone to the Canadian or Mexican border. Though technically crossing into New Mexico or Arizona would have also achieved this as they were still territories, Arizona did not become the 48th state until 1912.