When Morse knocks on Mrs Fairford's door, the door knocker is installed upside down.
The blue Rover 2000 car that has crashed into a lorry at the end of the episode has a registration number ending in L. That's a 1972-73 registration number, and the episode is set in the late 1960s.
Following the discovery of the husband and wife murder-suicide, the three police officers huddle at the police station. A shadow from crew or equipment movement flashes across the shoulder of one of the two officers standing side-by-side while facing the third officer in the wide angle shot.