The Rolls Royce is shown being spray-painted in the owner's garage. The garage should have over-spray everywhere, especially on the floor. There should also be the smell of the paint lingering in the air.
After several days bouncing around in the trunk of a car, the murder victim is surprisingly good condition, as seen at the end of the show. No discoloration or rigidity.
After the car is painted, it is driven before the paint has time to dry.
Mr. Wescott makes no effort to keep his fingerprints off of the envelope that he mailed.
For reasons unexplained, the police don't trace the phone call from General Delivery back to Philadelphia Harry's place. This is especially obvious since later, when Westcott makes a phone call from a phone booth, the detective says he didn't call from his home for fear of it being traced.