The Gemara introduces a case where the Lehi (side post) – the object that is placed at the entrance to a Mavoy (alleyway) as a reminder that carrying can only be done within the walls of the Mavoy and not in the public domain (see 5a-b) – appears to be an extension of the wall of the Mavoy, so that it is only apparent from the outside that it is a Lehi. The amora, Rabba bar Rav Huna, argues that it should be considered a valid Lehi, but, basing itself on a baraita that appears to contradict that position, the conclusion of the Gemara seems to be that such a Lehi is not valid.
Rav Yosef, who was a student of Rav Huna, comments that he had never heard the position taken by Rabba bar Rav Huna that such a Lehi should be valid. It should be noted that during an illness, Rav Yosef had forgotten much of his learning (see Nedarim 41a), so his student, Abaye, reminded him that he had, in fact, quoted Rav Huna as ruling that if the inner wall of the Mavoy extended beyond the Mavoy less that four amot (cubits), then it is considered a valid Lehi. Furthermore, Rav Yosef himself had concluded from that statement that a Lehi is valid if it can be seen from the outside of the Mavoy, even if it cannot be seen from inside the Mavoy.
Following this exchange, the Gemara concludes that such a Lehi is considered valid, contradicting the earlier supposition of the Gemara. To explain the change of ruling, the Gemara says that they prefer to rely on the baraita that was quoted earlier (9b) in the name of Rabbi Hiyya.
The Jerusalem Talmud explains that the baraita upon which the first ruling was based could not be found recorded in the authoritative collections of baraitot, and therefore could not be relied upon. The baraitot quoted in the study halls of Rabbi Hiyya and Rabbi Oshiya, on the other hand, were known to be reliable.