Revealing mistake: Paul Lynde is supposedly "in" the mirror near the front door, but as he waves his arm to cast a spell, the shadow of his arm waving is visible at the far left, revealing he is actually standing off camera at a spot where his reflection is visible. Elizabeth Montgomery also briefly glances at him. (00:02:25)
To Trick or Treat or Not to Trick or Treat - S6-E7
Revealing mistake: The wire used to control the flaming arrow's flight is visible in the closeup. (00:21:00)
Samantha's Yoo-Hoo Maid - S6-E2
Revealing mistake: Whenever Esmeralda sneezes and makes the furniture in the living room levitate you can see the wires.
Revealing mistake: After Samantha restores The Leprechaun's power he walks into the backyard, and when Endora, who's on the roof, uses her powers to have him float up to join her, the wire cables can be seen giving him the needed boost upwards. Later, the cables can be seen again when Sam lifts him up in the living room. (00:09:40)
Serena Stops the Show - S6-E22
Revealing mistake: The face of the double can be seen when Samantha and Serena come from the kitchen. (01:10:00)
Answer: Telstar was an early communications satellite system. Darrin, in a particularly sexist manner reflective of this era, is saying that telling a secret to a woman means it is guaranteed to be "broadcasted" to multiple people.
raywest ★