Terumos, Chapter 8, Mishnah 1. This Mishnah addresses the following question: what is the law concerning someone who ate Terumah thinking that he was entitled to eat it, and only afterward learned that he had not been permitted to do so? We are dealing with an innocent mistake: the person believed that he had a connection to a kohen, or that he himself was a kohen, and in the end it turned out that this was not the case. The Mishnah considers several cases, and we will take up each one on its own.
"Ha'ishah shehaysah ocheles biTerumah" - a woman who was eating Terumah:
This is a woman whose father is not a kohen, but who married a kohen. As part of the kohen's household, his wife is certainly permitted to eat Terumah. But then people came and told her: "Meis ba'aleich o geirshech" - your husband has died, or he divorced you. If her husband died and she has no children, she reverts to being considered part of her father's household and is no longer attached to a kohen; she is no longer entitled to be part of his household, and in fact there is no household - there is no longer a kohen. Consequently she is no longer permitted to eat Terumah, and since she did eat, the question is whether she must pay the principal plus a fifth. The answer, on the plain level, is that she must indeed pay.
"O geirshech" - or he divorced you. At first glance it is puzzling how a woman could not know that she had been divorced, since a woman is not divorced against her will. But the case is one where she sent an agent to receive her get on her behalf: she knew that she was about to be divorced, and she assumed that some two weeks would pass until the agent made his way there and received the get. In practice the agent completed his mission within a week, and word was sent to inform her that the get had already been received on her behalf, earlier than expected. It emerges that she ate Terumah by mistake, thinking she was still married, when in fact she was already divorced - and here too the same result: she must pay the principal plus a fifth.
"Vechein ha'eved shehayah ochel biTerumah" - and likewise a slave who was eating Terumah:
This refers to a Canaanite slave, who is considered part of the kohen's household since he is his property, and for that reason is entitled to eat Terumah. While he is eating his Terumah, people came and informed him of one of the following:
"Meis rabach" - your master has died, meaning that he is no longer part of that household. Normally the owner's son would inherit him, and since the son is also a kohen, nothing changes. But there are scenarios in which the sole heir is the kohen's daughter who married a non-kohen and thereby left the priestly family, and the slave of such a woman is not entitled; and likewise if the sole heir is his grandson through a daughter who married a non-kohen, since that grandson is not a kohen. In these cases the slave became part of the household of a non-kohen, and he was not entitled to eat Terumah.
"Mecharcha l'Yisrael" - your master sold you in the meantime to a non-kohen, and you are therefore part of the household of a non-kohen, who has no right to Terumah.
"Nesancha b'matanah" - he gave you away as a gift; and if it was not a sale but merely a gift, the law is the same.
"Asa'cha ben chorin" - he made you a free man. A slave can be freed, and even through an agent. And unlike the divorce of a woman, since our Mishnah understands that freeing a slave is a great benefit for him, he need not consent to it at all: the owner frees him even without his knowledge, and the emancipation takes effect. So it turns out that they brought good news to the slave while he was eating Terumah, that the kohen who had been his owner had freed him and he is now a free man; but now it emerges that he ate Terumah unlawfully, and he must pay the principal plus a fifth as compensation.
A kohen who learned that he is the son of a divorcee or the son of a chalutzah:
This is a kohen eating Terumah, thinking that he is permitted to do so by virtue of being a kohen, and then he learns that a ruling was issued in his case that his mother had been a divorcee or a chalutzah - a woman with whom the chalitzah process was performed, who for this purpose is also included in the category of a divorcee. The halachah is that a kohen may not marry a divorcee or a chalutzah, and if a kohen did marry a divorcee, the child is called a "chalal" - from the word meaning emptied out, since such a person has been emptied of his priesthood. And a chalal may not eat Terumah. It emerges that this person who ate the Terumah, and then learned that he is not a kohen but a chalal, ate it in a forbidden manner, and he must pay compensation - the principal plus a fifth.
The Mishnah therefore rules straightforwardly that in all of these cases - the woman, the slave and the kohen, in whose cases it was discovered that they had eaten Terumah unlawfully - "Rabbi Eliezer mechayeiv keren v'chomesh, v'Rabbi Yehoshua poter" - Rabbi Eliezer obligates them in the principal plus a fifth, and Rabbi Yehoshua exempts them. According to Rabbi Yehoshua, in all of these scenarios - the woman, the slave and the kohen - they are all exempt from paying the principal and the fifth. In truth, however, Rabbi Yehoshua does not disagree with Rabbi Eliezer in principle, and his dispute applies only to one particular, exceptional case.
Rabbi Yehoshua is speaking of a case in which the eating of the Terumah took place on Erev Pesach, in a moment of great pressure, while they were occupied with the mitzvah of destroying chametz. Terumah that was chametz was lying before them, and they said: we have only one hour left in which to eat it, and they swallowed it down. They thought they were engaged in a matter of a mitzvah, and their error came about in the course of performing a mitzvah, thinking that they were doing a mitzvah when in fact it was not one. In this case, and only in this case, Rabbi Yehoshua says that they are exempt from paying the principal and the fifth.
The halachah follows Rabbi Yehoshua. And what that means is this: in any ordinary scenario, even if the person thought he was eating Terumah lawfully and in the end it turned out otherwise, he is obligated in the principal plus a fifth. But in the exceptional scenario, where it was Erev Pesach and he ate it only in order to be rid of the chametz that was there, and it turned out that he had not been permitted to eat it in the first place, so that he erred in the course of a mitzvah - he is exempt.
A kohen who was standing and offering sacrifices on the altar:
From here the Mishnah moves to a related subject - not eating Terumah, but the service of the kohen: "hayah omed u'makriv al gav hamizbe'ach, v'noda shehu ben gerushah o ven chalutzah" - he was standing and offering on the altar, and it became known that he was the son of a divorcee or the son of a chalutzah. A kohen is actually performing the service and offering a sacrifice on the altar in the Beis HaMikdash, and at that very moment, one moment too late, he is informed that he is not a kohen at all but a chalal, since his mother was a divorcee or a chalutzah. What is the status of the sacrifices he offered? Rabbi Eliezer says: "kol hakorbanos shehikriv al gav hamizbe'ach pesulim" - all the sacrifices he offered on the altar are invalid, for he was never fit to offer sacrifices, being a chalal, and consequently all his offerings are disqualified. And from here, whatever the consequence may be: if the owners need to bring new sacrifices, they will bring new sacrifices, since these offerings are invalid.
Rabbi Yehoshua declares them valid, and the halachah follows his view: in such a case, after the fact, the offerings of this kohen who was in truth a chalal are valid, and whoever brought them has fulfilled his obligation. Rabbi Yehoshua's reasoning is rooted in a verse. There are different sources as to which verse is meant, but whatever the derivation may be, on its basis Rabbi Yehoshua understands that when a chalal has offered a sacrifice, after the fact his service is accepted.
On the other hand, "noda shehu ba'al mum" - it became known that he was blemished. Here we are dealing with a kohen who is informed that he is indeed a kohen, but that he has a blemish which disqualifies him from serving in the Beis HaMikdash. This man had no idea of it at all, and now he is told that the very defect no one had noticed until now renders him a ba'al mum, and therefore his service is not valid. Here all agree that his service is invalid: since he was not fit as a kohen to offer sacrifices and serve in the Beis HaMikdash, everything he offered while he was blemished is retroactively disqualified.
In summary: in this Mishnah we learned the law of one who eats Terumah by mistake - the woman who learns that her husband has died or that he divorced her, the slave who learns that his master has died, that he was sold or given to an Israelite, or that he was set free, and the kohen who learns that he is the son of a divorcee or the son of a chalutzah. Rabbi Eliezer obligates payment of the principal and the added fifth, and Rabbi Yehoshua exempts, and his dispute is limited to a case in which Terumah that was chametz was eaten on Erev Pesach through a mistake made in the course of a mitzvah, where the halachah follows him to exempt. We also learned the law of a kohen who was standing and offering sacrifices and it became known that he was the son of a divorcee or the son of a chalutzah, where Rabbi Eliezer invalidates his offerings and Rabbi Yehoshua declares them valid after the fact, and the halachah follows Rabbi Yehoshua; whereas when it became known that he was blemished, all agree that his service is retroactively invalid.