Continuity mistake: When Helen Grace is in the retired GP's house choosing which apple she will eventually stuff in to her victim's mouth, her hand wavers over one apple, but instead chooses another. At the end when we see a flashback of that scene, she picks up the first apple she was originally going for.
Factual error: Whenever the killer (always dressed in black with a balaclava disguising his/her identity) uses a knife, the blow invariably is an overhand strike. However, a knife in a downward thrust quite frequently simply skids off the overlapping ribcage of the victim.
Plot hole: Several shots show Barnaby reviewing a newspaper announcing a new death in Strangler's Wood, the Brazilian Woman. No one supposedly knows who she is, as the article states, but the photo of the dead woman is a professional headshot from when she was alive. How could they have obtained her headshot if no one knows who she is?