Horayot 4 - After Court Retracted
If the court ruled incorrectly and permitted something that the Torah has prohibited under the pain of excision - being cut off from the people - and the majority of Jews did it, then when the judges realize their mistake, they need to bring a bull sacrifice, to atone for the "sin of forgetfulness."
What happens if an individual did it because he has not heard that the court has retracted? Rabbi Shimon always exempts him. Rabbi Elazar says that his error is in doubt, and therefore he must bring a special guilt-offering. This applies when he stayed at home and could check on the court opinion before doing the act. If he went overseas, he is completely exempt.
The court needs to forget only part of the law, but not the complete law. For example, if they ruled that there is no Shabbat in the Torah, they are exempt from the bull offering, but if they said, "There is Shabbat in the Torah, but one is allowed to carry things from one's house into the street," - then they are liable to a communal-error bull offering.
Art: Pierre-Auguste Renoir - Man Carrying a Boy