The whole premise of the episode rests on Larry not believing that Sam has an identical cousin. The thing is, Serena was revealed in an earlier Season 4 episode, Episode 10, "That Was No Chick, That Was My Wife," when Louise Tate was considered crazy for seeing Samantha at home when she was out of town with Darrin. Louise was about to be sent to a psychiatrist when Serena saved the day by claiming it was she who Louise saw. Louise would have made a big, huge, major deal of this to Larry with, "See? I'm not crazy!" to the point that Larry wouldn't have forgotten it at all, let alone so soon, making the entire episode not work. Otherwise cute.
Continuity was not a strong point of 1960's television. The thought was that an episode might be seen just maybe a couple of times after its first broadcast then seen no more. Soon to be forgotten permanently. Funny thing was some shows had a staffer to track plot points in order to be coordinated for future stories but evidence would suggest that they either did not do their job or were readily ignored.
Some of those continuity people should have been shot, that is if they were the ones making the decisions. Not only were they ignored, so generally were actors who pointed out that something written for their character was just wrong but in some cases were still forced to do it. These things are glaringly obvious when streaming a series or watching on DVD so you see it all in a short amount of time. Currently watching Little House on the Prairie which was one of the worst offenders.
I would put most of the blame on the producer and head writer. In those days many times a show was only a few episodes ahead of what was being aired for a given week. The production staff often found itself in a predicament in coming up with the story for the next episode to be produced.
I have a Bewitched companion book. Just the other day I was reading it (again) and the book points out that same mistake in "Hippie, Hippie, Hurray."
If you pay attention, you'll find a number of continuity errors in the dialogue. For instance, when Louise confides in Sam that she is finally going to be a mother, she asks Samantha if she knows that she and Larry have been married for sixteen years.
Yet a few seasons later in "Bewitched, Bothered and Infuriated", Darrin insists on intruding on the Tate's second honeymoon. He mistakenly thinks Larry is going to break his leg on the trip. Larry gets fed up with Darrin's intrusion and tells him that "ten years ago" when they set out for their honeymoon, he broke his leg.
By that point, the Tates were married almost twenty years.
I also can't figure out just WHERE Darrin's parents lived! In the first b&w episode when they meet Sam, Darrin goes to the airport to pick them up. A few episodes later they come to dinner and Endora is also there. She is mildly flirtatious with Frank Stephens and Phyllis goes crazy with jealousy. Darrin goes over to their house to speak to his mom. There was never any mention of them moving close by in "Samantha Meets the Folks".
Sometimes they live nearby. Sometimes they live far away or so it seems. When they meet Tabitha, their first grandchild, she's already several months old. You'd think if they lived close they'd drop in right away for a visit.
Also, in the very first episode after Samantha tells Darrin she is a witch, he lists the things she will have to do. He says she'll have to learn to cook and keep house and have dinner with his folks every Sunday. That would be kinda hard to do since they flew in to see Darrin. Were they expecting the newlyweds to fly out to visit them every Sunday? lol
But as Farmer Ted points out, episodes were expected to be seen a few times and certainly not binge watched. I tend to do that with some shows like Bewitched and the discrepancies are easier to spot when you watch a bunch of episodes in a row.
Back in the 1970's while syndication was the only way to see these shows you might see an episode but then not see it again until a year or two later. Plenty of time for the memory to dim.
The craziest thing about this episode, is that it DOES acknowledge that Louise and Serena have met before. After Larry invites himself and Louise to dinner at Samantha and Darrin's so they can see Serena for themselves, Serena files off in a huff after Darrin insults her, and refuses to stay for dinner. Samantha shows that she can pose as Serena for the evening. The only problem is that she can't be both Samantha and Serena at the same time.
When the Tates arrive, as Darrin is greeting them and taking their coats, "Serena" (Sam in disguise) enters the room and Darrin says, "You remember Louise Tate, don't you, Serena?" Samantha/Serena responds, "Oh sure!" Then, to Larry: "Hey Cotton Top that's a wild looking chick you've got there!"
Still, as you've pointed out, it does leave a big plot hole unresolved. Why didn't Louise just say to Larry something like:
"Larry, that could be Samantha's cousin, Serena. Remember the mixup we had when Sam and Darrin were in Chicago and I stopped by their house and thought Samantha was at home? It turned out to be her cousin, Serena, and they're almost identical. We should make sure it isn't Serena in the paper before you start jumping to conclusions."
Of course, we all know that if she had, there wouldn't have been an episode, but other than hat, it defies logic. lol
I am just overthinking this episode! But upon reflection, I don't know why it would have been a big deal even if it WAS Samantha who took part in a hippie peace protest.
What I mean is that Darren was hardly a celebrity. He was one out of thousands of executives working in New York. No one would really know that Sam was his wife.
And come on. This was New York. Those folks are pretty sophisticated about lots of things. I don’t think they would be scandalized. It’s not the same as living in a tiny conservative village where everyone knows you and your business!😊
I guess it was just supposed to be that Larry's latest potential cash cow client, "Mr. Giddings," was so conservative that HE wouldn't understand Samantha's turning hippie.