Horayot, Chapter Three, Mishnah Three
The first section of this mishnah discusses a situation where the high priest or king issued an errant ruling before they were appointed to their position.
Section two defines the word which we have been translating as ruler. Leviticus 4:22 employs the word nasi which is translated occasionally by chieftain or prince. In the Torah, it does not usually refer to a king. However, our mishnah defines it as a king, as we shall see below. It is worthwhile to note that in the times of the Mishnah, there was an official who used the title nasi, usually translated as the patriarch (for example Rabbi Judah Hanasi). However, the patriarch certainly did not have kingly powers.
I) If they transgressed before they were appointed, and afterwards they were appointed, they are regarded as regular people.
A) Rabbi Shimon said: if their sin came to their knowledge before they were appointed they are liable, but if after they were appointed they are exempt.
II) Who is meant by a ruler? A king; for it says, Any of all the commandments of the Lord his God (Leviticus 4:22), a ruler (king) who has none above him save the Lord his God.
Section one: If the priest or king issued an errant ruling before they were appointed, they are not liable for the special sin offering, since at the time of the ruling they were regular individuals. As we learned above, their liability is dependent upon the time of the ruling. In this case, if they transgressed they are liable to bring the same sin-offering as would a regular Israelite.
Rabbi Shimon holds that if they found out they had erred before their appointment they are liable to bring a sin offering as would a regular Israelite. However, if they found out about their sin after they were appointed, they are completely exempt. Rabbi Shimon holds that their liability is dependent both upon the timing of the errant ruling and the realization that it was errant. In order for there to be liability, the high priest or king must be of the exact same status at the time of the realization that he was at the time of the ruling and sin. Evidently, Rabbi Shimon would disagree with the opinion given in the previous mishnah as well, that if the high priest or ruler issued an errant ruling and then relinquished their office they are still liable. According to R. Shimon, if they realized that the ruling was errant after they left office, they would not be liable.
Section two: As stated above, according to the mishnah the word nasi, which we have been loosely translating as ruler, really refers to the king. This is learned from the phrase, the Lord his God. Only the king is second to God.
Note, the simple meaning of nasi in Leviticus is probably head of a tribe. That is the usual usage of the word in the Torah (see for instance Numbers 7).