Revealing mistake: When Sam and Endora are sitting on the wing of the airplane, you can see folds in the blue backdrop for the sky.
Deliberate mistake: The drunk that is always in the bar in New York is the same drunk in Salem that sees Sam twitch the witch on the signs from an old hag to a young witch resembling her.
Answer: Larry had been telling Darrin what it means psychologically that Samantha gave the coat away. Larry said giving it away meant she's happy being married but if she liked the coat it means she desires to be single and carefree. So when Darrin sees the coat, he immediately thinks Samantha doesn't want to be married and the fact that she twitched it means (in his mind) she wants to be a single, carefree witch again.
Bishop73