Opening shot shows Strachan's easel facing inland when he was painting the Isles of Fleet.
Strachan has his back to Ringdoo Point but, when he walks towards Wimsey and sits down beside him, he is advancing in the opposite direction towards Ringdoo Point.
When Peter is tracing the murderer's route on a bicycle, as he begins his journey he attempts to put a piece of paper (a map?) into his pocket. It does not go into his pocket and is clearly seen to fall to the ground as he cycles away from the camera. This is quite unintentional and has nothing to do with the plot.