Continuity mistake: At the lake, when the Doctor stands in front of Agatha and tells the Vespiform, "You're not meant to be like this! You've got the wrong template in your mind!", in the close-up shot he has his left arm raised, but in the Vespiform's POV shot he has his right arm raised.
Continuity mistake: When Robina Redmond is announced, and Lady Eddison says, "She's the absolute hit of the social scene - a must," in the first shot, Lady Eddison is standing close enough to her wheelchair-bound husband to cast a shadow on him. In the second shot, the distance between them has suddenly increased.
Continuity mistake: After questioning the houseguests, the Doctor and Agatha are talking. She uses the phrase "little grey cells", and the Doctor responds, "Oh, yes, little grey cells. Good old Poirot." When he does this, he's holding a saucer in his left hand and rests a teacup he's holding in his right hand atop it, as he turns toward the chair that the suspects were sitting in. In the next shot, as he walks over and sits in the chair, he is suddenly holding the teacup and saucer in his right hand, with his left hand by his side.
Continuity mistake: After explaining that he's laced the soup with pepper, the Doctor, when he says, "Anyone got the shivers?", turns his head from looking to his left so he's looking in front of him. In the following wide shot, his head is turned to his left again.