Deliberate mistake: After Barney buys the car from Mrs. Lesh, he and Andy are standing at the steps as he tells her not to look back at the car, and the next shot of Andy and Barney at the front door is the same shot from a few moments prior, but it's been flipped so they would be looking in the opposite direction. (00:09:30)
Visible crew/equipment: While Andy and Barney are hiding behind the car, when Mrs. Lesh hits Jake in the stomach with her umbrella, we can see two curved chalk actor's marks and two chalk T-marks on the floor. (00:22:10)
Answer: Those "boxes" are valences that used to be very common, before air conditioning. They allow for windows to be open during rain storms. They permit air circulation, without letting the rain in.