Continuity mistake: At the start, Opie and his friend rush to the front door of the same house to try and sell their products - note the house next door (and the fact that it's three houses down from Andy). The kids then run across the street and have the door slammed in their faces, but then it cuts to the woman next door as she walks to her front door - note the house behind her is that first house Opie went to, even though all the boys just crossed the street a few moments ago.
Visible crew/equipment: At the end, when Barney's talking to all the boys at the courthouse, the camera angle's looking up in Barney's closeups, and the top of the courthouse set is visible right before Barney mentions "intimidation."
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.