Revealing mistake: When the first scene at Emma's hideaway was shown, obviously the two characters Emma and Jessica were filmed seperately on the couch and then the film was put together (as both are played by Angela Lansbury). However Jessica did a lot of moving while she was talking, so Emma was looking straight across at where she had been, despite her bending down while she talked.
Revealing mistake: There are 2 scenes involving Jessica inside a car. In the first one the car drives calmly as seen from the outside shots, but from the shots inside the car is speeding. When the episode is about to end the car speeds to the police station and now the exact same projection is played, though this time it makes sense.