פירוש על הוריות 1:2

Rabbi Shimon exempts him – Because the ruling had reached the majority of the community.

If a court ruled, and later discovered that they had erred and changed their decision, whether they brought their offering or whether they did not bring their offering, if an individual proceeded and acted in accordance with their [erroneous] decision, Rabbi Shimon exempts him and Rabb Elazar declares [his case] doubtful.
Which case may be regarded doubtful? If he was at home, he is liable. If he went abroad, he is exempt.
Rabbi Akiba said: I agree that a person in such a case is nearer to exemption than to culpability. Said Ben Azzai to him: how does such a person differ from one who remains at home? He who remains at home is in a position to ascertain the facts but the other was not in such a position.
This mishnah discusses a case where a court made a wrong decision and then reversed their decision, thereby correcting it. The question asked is whether or not one who follows the wrong decision after it has been revoked is exempt from bringing a sin-offering for his accidental transgression.
Section one: The offering referred to in this section will be discussed more fully below in mishnah five, specifically the question of who brings the sacrifice.
Rabbi Shimon exempts this person because he relied on the court. As we learned above in mishnah one, a person who relies on the court is exempt from bringing an individual sin-offering. Rabbi Elazar declares this case doubtful, for we do not know if he had already heard that the court had reversed its decision.
The mishnah now continues to discuss in which specific cases the matter is in doubt. If the person was at home and could have known that the court reversed its decision, he is responsible for not having followed the new decision. However, if he had gone abroad, he could not have known of the reversal, and he is therefore exempt.
Rabbi Akiva states that this person, who went abroad, is actually closer to being totally exempt, for it is very unlikely that he knew that the court reversed its decision. As he explains to Ben Azzai, his colleague, the person sitting in his house could have heard, whereas the one who was abroad could not have.

Rabbi Elezar declares this case doubtful – Since he should inquire any time there are new rulings by the beit din and he didn’t inquire, this is similar to the case where he is in doubt whether it is a sin or not and he brings an asham talui. The law is according to Rabbi Elezar’s opinion.

Which case may be regarded doubtful – For example, like what Rabbi Elezar said, the matter being discussed is that he was satisfied with the ruling, whether a sin or not a sin and he is obligated to bring an asham talui.

One who remains at home – When a person is at home in the country where the beit din ruled, he could have heard that the beit din reversed its ruling.

If he went abroad – This does not necessarily mean he already set out, but if he was preparing to travel but had not yet left, Rabbi Akiva holds the opinion that a person who is preparing for a journey will not be able to inquire whether the beit din reversed its ruling and he is exempt from bringing an asham talui. Ben Azzai thinks that since he had not yet set out on the journey, he should have inquired. This is the disagreement in the Gemara. And the law goes according to Rabbi Akiva’s opinion.