Continuity mistake: There is no way the cow (a Hereford I think) could have given birth to that calf (a Jersey or something like that). [The mom is a mixed-breed BEEF cow, the calf a purebred Jersey DAIRY calf. An amazing feat of animal husbandry.]
Visible crew/equipment: When the young lady is trying to drag her dead husband away to bury him, as she takes a hold of the blanket you can see the covered wagon in the background and someone's legs behind the wheel begin to move to the right. She was suppose to be alone except for her dead husband. (00:34:00)
Factual error: When the riders are watching the wagon train, in the background you can see a communication tower with conical speakers.
Revealing mistake: As Jango fires the machine gun, the ammunition chain hanging from it is loaded on both sides, and it doesn't move. Also the belt has different sizes of cartridges in it. (00:45:30)
Other mistake: Bandoleer rounds don't fit any weapon in the show. When Pierce tries the Gatling gun, there is no magazine in it.
Factual error: Outside the Territory Hotel, during the storm, and once earlier in the movie, the taxi cabs are shown with the modern "flip over" vacancy/hired signs that were not used then.
Continuity mistake: In the first shot of the dead man hanging form the tree, his head appears to be about 12" from the tree limb. After Mattie climbs up to cut him down, his head is several feet from the tree limb.
Continuity mistake: When the gang of bad guys ride to the ranch there are nine of them. When they leave, and throughout the rest of the movie there are only seven, despite only one being shot off his horse at the ranch. (00:15:15)
Other mistake: There's a horse that has apparently ridden up a cliff, and an electrical burning brand that can be changed onto different flame settings.