daf bit: Eruvin 62

There is a halacha (as I understand it) that a student or disciple is forbidden to issue rulings within the jurisdiction of his master (for example, in his master's district). The g'mara raises but does not here answer some questions about this. We have a tradition, Abaye said, that the teaching of R. Eliezer b. Yaakov is small in quantity but well-sifted, meaning that few of his teachings are recorded but the ones that are are always right. So, Abaye asks, can a disciple rule in accordance with him even in the jurisdiction of his master? R. Yaakov b. Abba asked Abaye: may a disciple give a ruling that is authoritative as Megillat Ta'anit (the only written halachic collection at the time)? The g'mara says that R. Hisda made no ruling on either case and does not seem to resolve the question. (62b)

My intuition would be that even if the answer is "obvious", it's not considered proper -- because you might be wrong about obviousness. Also, note that we are talking about the narrower case of making rulings, not the broader case of teaching.