During the opening, Gerald Hadley's voice is reading out what he is writing. He says he fired 2 35mm rounds which missed by 2 inches etc. A 35mm round is huge and the size of round used in an anti aircraft gun, and not the size to be fired from a hand gun or machine gun.
Russian Blue cats tend to be hypo-allergenic. It's highly unlikely anyone who did not know they were allergic to cats would react to a Russian Blue for the first time in their life.
The corpse of Max Jennings is breathing.
Honoria Lyddiard is pronounced dead but is still breathing as well.
When Barnaby and Troy are driving to Honoria's house for the last time, it appears to be raining heavily, and yet the sun is clearly shining in on Barnaby's face.
The talk to the writer's group is the evening of 1st July. When they're trying to find the taxi which dropped off a woman at the house the night before the talk, DCI Barnaby tells Sgt. Troy to contact cab companies and find out about "the 31st." June only has 30 days.