At 21 minutes, Samantha standing behind the couch casts a spell on the Darrin's wristwatch to fall. Cynthia Pennybaker with crossing her legs picks it up. Right in the next distant shot, Samantha is already in front of the couch and Cynthia is not crossing her legs.
Near the end of the show, Darrin is telling a story and the camera is zoomed out a little -he is clearly not wearing a watch. The next moment when the camera zooms in he is wearing a watch (pivotal to the scene)
At the beginning of the show, Darrin is wearing his watch on his right hand. After his meeting with Pennybaker, he goes into his office and takes his watch off his LEFT wrist. It's also on his left wrist in the living room scene where the watch falls off. It's uncertain why it was on his right wrist earlier. A right-handed person typically wears a watch on their left wrist so they can operate its controls with their dominant hand.