Visible crew/equipment: When Jessica is on her horse at the barn, there's a crew member lying on the floor, watching out for Angela Lansbury.
Revealing mistake: After Denton falls off his horse, Jessica arrives and jumps off hers. Her stand-in is a much younger and thinner woman who looks nothing like Angela Lansbury.
Answer: The storyline was that Jessica was teaching classes in New York. However, after so many seasons and as ratings declined, it was probably to freshen up the series with a different locale, introduce new characters, and have a greater variety of situations in which Jessica could solve murders. It was already unbelievable that so many people could be murdered in the small town of Cabot Cove. Being in a large city like New York makes it a tiny bit more plausible.
raywest ★