The American flag hanging on David's front porch is the 50 star version that is currently in use. However there were 48 states at the time the movie took place (1958), thus only 48 stars on the American flag.
When David goes to talk to his Father at work. There are several Liquor Bottles sitting on the Bar that had UPC symbols on them. UPC symbols didn't come out until the late 70's or very early 80's. The story takes place in 1958 way before UPC symbols existed.
In the basement when Meg is tied up and undressed by the boys on the order of Gertrude. The underwear she is seen wearing while appropriate for her age group (12-16) as she is 16, they would not have been manufactured until around the time of the films production and release.
When David is talking to his father about when it's okay to hit a girl, his father sets three bottles of alcohol out on the bar. There's a close-up as David stands at the bar and, when the camera pans out, the three bottles are gone without David having moved to put them away.
When David is sitting next to Meg on the bed while the police officer is questioning him, he has blood dripping down the left corner of his mouth from being beaten. When they show a close-up, the blood is gone and the dirt but when the camera switches back to the side view, it's back again.
The first time David goes down stairs he's wading through hanging laundry. When he goes through the last sets of linen another hand can be seen on the left helping him pull the laundry aside and hold it there, presumably, for the camera.
In the scene at the park where Meg is seen talking to Officer Jennings, on his police car it clearly is labeled "Highway Patrol". It is not a Highway Patrol officer's job to preside over such events, nor is it to investigate possible child abuse situations.