Ronald defeo wasn't the brutal man he was depicted to be in this film.
When Butch and Dawn are chanting to the spirits, there are battery operated candles used in the room. These candles are similar to modern ones used in place of real candles. They were not around in 1974.
At the 1:38:06 mark. In the kitchen, the two boys are fighting over a Stretch Armstrong toy. The time period is 1974, and Stretch Armstrong wasn't released until 1976.
Modern electrical outlets, not available in 1974, are seen in the kitchen.
After Dawn returns home to find her family murdered, she is followed by Butch to her bedroom. She forces a chair under her doorknob to keep the door shut.
1. A standing light is visible on the left hand side of the door. In following scenes it is missing
2. After she goes to the window, two following shots of the blocked door show a mustaches crew members head clearly in the bottom left.