The red, white, and blue ice pop (also known as a Bomb Pop) that Joshua is eating in the flashback keeps changing size.the red portion in particular gets bigger, then smaller. It completely disappears at one point, then returns.
When Brent delivers the "Sawzall" line, he has Fruitloops stuck to his face. Moments later when he answers the door to greet his parents, the Fruitloops are missing (likely because it was an idea they had after the door and ending had already been shot).
When Kendall is crying in her car she wears a seatbelt; cut to a different angle and she's not wearing it; when the film cuts to the front angle for the phone call she's still not wearing it.
The hinge to the basement is on the parents' side, so they could have taken the door off its hinges to get to their kids.
When Brent leaves for work, his car is parked in the center of the driveway although Kendall had left the house a few minutes earlier. This would have prevented her from exiting the garage or parking on either side of Brent's car.