Stupidity: Oscar, Dwight, Jim, Erin, Kelly and Kevin are worried about a dog that have been left in a car, and that the dog will die. After some discussion, Oscar ultimately decides to smash the rear-driver's windows in order to give the dog some fresh air, yet there would be no point in doing this as the sun-roof is open, that's how Dwight pours milkshake into the car.
Stupidity: Dwight is seen walking out of the bathroom with the diaper in the direction of the main part of the office in which Angela sits. If he was to try to stay hidden (which was obviously the whole point), he would have gone through the exit of the annex, waited until she was distracted/or hide it for later and Angela never would have seen. This was done because the plot required for Angela to notice but Dwight could have easily kept it hidden.