After Teddy was drugged and bit someone the lawyer tells them that they are positive that Teddy wasn't rabid. The only two ways to proof something like that is a ten day quarantine or by a post-mortem (after death) examination of brain tissue. Neither of these two things happened.
After Denton's fall, Jessica jumps off her horse and turns towards the camera; it is clearly a stand-in and not Angela Lansbury.
During Denton Langley's fatal jump, he cries, "Tally, ho!" with his mouth closed.
In the first party scene, spike marks can be seen on the floor as Jessica and Denton walk across the room.
After Jessica and a deputy look for a bicycle clip on the side of the road, the scene switches to where Jessica is on a horse and Echo rides up on her horse in front of the barn, you clearly see the complete head and upper torso of a man in a red shirt lying on the ground directly in front of Jessica Fletcher's horse.