The movie is set in the 1930s, at one point a character is loading a shotgun with plastic shotgun shells, but plastic shells weren't introduced until the 1960s. During the period of the movie shotgun shells were made of thick paper with a metal bottom for the primer.
Telephone (utility) poles shown in several scenes include both telephone lines and cable TV lines, some of which include modern "expansion loops." Cable television wasn't introduced in the US until 1948.
Early Galvin radio in dash of the '32 Ford V-8 was not correct because at that time radios had a separate control head with the radio mounted under the dash. Also, the horizontal dial was not accurate to this time.
When Jack grows ill while Bertha washes his foot, and out of the church, he leaves a boot behind. He is seen limping through the woods wearing a single boot. When he arrives at Cricket's cabin and is beaten outside the cabin by Charlie Rakes, he is shown wearing two boots. In the next scene, back a the Bondurant's Inn, Forest gives Jack a bag (a "present" from the preacher, Bertha's father). In the bag is the boot he left at the church.
When Floyd Banner is introduced, he shoots down several men in a car in the center of town with a Thompson submachine gun. Near the end of the scene the rear of the car is shown with bullet holes in it, but Floyd Banner was never behind the car, and he could only have put holes in the side and front of the car.
Heavy kudzu growth is visible in many scenes - this is an invasive plant species introduced to the United States in the early 20th century, and it's much more common in Georgia where the film was shot, than Appalachian Virginia where the film was set. And it probably wouldn't have been in either of those areas at all in the 1930's.
The flowers at Cricket's funeral (ranunculus, wax flower, French anemones) would likely not have been available in the rural mountains of Virginia at the time. They would have likely used wildflowers such as goldenrod and queen anne's lace.
The goof item below may give away important plot points.
During the shootout at the bridge, the sheriff shoots Agent Rakes, seemingly in the back of his left leg. But when Rakes retreats to the bridge, there is no sign of a bullet-hole or blood anywhere on the back of his suit.