Passover

To whom this may concern, When Passover falls out on a Saturday Night, the Havdalaha service is added into the Kiddush section of the Haggadah in accordance with the Halacha of "Yak’n’haz" (the yayin, kiddush, nar, havdalah, z’man rule), just like Pesach was observed last year in 2008. I recently heard that a long time ago, during the time of the Karaites I think, this Halachah was actually forgotten and Jews did not know what to do until someone remembered the Halchah from I think Bavel. I’m pretty sure this episode was recorded in the Talmud but I’m not sure where. Assuming all my information is correct, I want to understand how Jews who cared so much about the Halachot could forget such an important Halachah about the observance of Passover on Saturday Night. Obviously, something on the national level was happening that could cause such a problem and this is what I want to know. I heard that this author Lawrence Shiffman once proposed an idea that the law was forgotten because the Jewish calendar was incorrect or something. I have no idea if this is correct though. Thank you Chag Sa’meach

Your questions are very good assuming the story is true, I however could not recall such a story in the Talmud, if this story never occurred the question is solved. If this story does exist somewhere it might be in a certain place where the tradition was cut for some reason or another but not lost from the Jewish people as a whole.