Revealing mistake: The gang's visit to Northfield, Minnesota reveals a typical late 18th century town, with dozens of people on many horses and horse drawn wagons. All the horses leave deep tracks in the dirt, because they are walking on freshly laid soil that has not had time to compact. There is absolutely no horse manure to be seen anywhere, despite dozens of horses. Clearly, the film makers used a 20th century town and laid new soil over the pavement shortly before the filming began.
Revealing mistake: When Bud and Wes take competitive turns riding the bull at the presumed high speeds, the bucking patterns and timing aren't consistent. A few times either man goes less than 15 seconds on a mid speed rather than full on bucking. Which even for a practice doesn't add up, and wouldn't fly during a competition.