The Gemara discusses the case of a “Mavoy Akum” (= a crooked alleyway), which Rav considers “open” so that it cannot be treated like a normal Mavoy, and Shmuel considers “closed” so that the usual side post or cross beam can be used to permit carrying in it.
There are several explanations of what the “Mavoy Akum” looks like. According to Rashi, the Mavoy has an “L” shape and both ends are open to public domains. The Meiri suggests several other possibilities:
The Mavoy has a “C” shape so that both ends open to the same public domain.
The Mavoy has a “T” shape and all three ends open to a public domain.
The Mavoy has an “L” shape, but at an angle, so that both ends open to the same public domain.
The Gemara then tells the story of a “Mavoy Akum” in the Babylonian city of Neharda’a, with regard to which the community accepted two stringencies and ruled that only an actual door would permit carrying. On the one hand, they accepted Rav’s position that the Mavoy was considered “open” and that the typical side post or cross beam would not suffice to permit carrying in it. On the other hand, they accepted Shmuel’s position that an “open Mavoy” needs real doors and cannot be permitted with a simple tzurat ha-petah (=doorframe).
The Rif explains that the people of Neharda’a were forced to accept this position because the Halakha follows Rav in this case, but since Neharda’a was Shmuel’s hometown, they wanted to honor his position by following his opinion as well.
Nevertheless, the Gemara objects that it is inappropriate to follow two contradictory stringent opinions. A baraita is quoted which says that a person can choose to follow either the position of Beit Shammai or of Beit Hillel, but if he chooses to follow the lenient positions of both, he is a wicked person. If he follows the stringent positions of each, based on the passage in Kohelet (2:14), he is considered a fool.
When this story appears in a parallel Gemara (Tractate Rosh HaShana 14b), Tosafot explain that the passage in Kohelet refers not only to the person who does not know which position to follow and chooses the stringent position in all cases, but also the person who knows the correct ruling in each case but chooses to accept stringencies upon himself.