Yevamot, Chapter Eleven, Mishnah Five
This mishnah deals with a case where a priests wifes child is mixed up with her slaves child [this would have made a good TV movie]. Each kid doesnt know whether he is the son of a priest or the son of a slave.
1) If the child of a priests wife was mixed-up with the child of her female slave, behold both may eat terumah and [together] they receive one share at the threshing-floor. They may not defile themselves for the dead nor may they marry any women whether these are eligible [for marriage with a priest] or ineligible.
2) If when they grew up, the mixed-up children freed one another they may marry women who are eligible for marriage with a priest and they may not defile themselves for the dead. If they defiled themselves, they do not receive the penalty of forty lashes.
3) They may not eat terumah, but if they did eat they need not pay compensation either for the principal or [the additional] fifth. They are not to receive a share at the threshing-floor, but they may sell [their own] terumah and the proceeds are theirs. They receive no share in the holy things of the temple, and no holy things are given to them but others do not take their holy things from them. They are exempt from [giving to any priest] the shoulder, the cheeks and the maw, while the firstling of either of them should remain in the pasture until it contracts a blemish.
4) The restrictions relating to priests and the restrictions relating to Israelites are both imposed upon them.
Section one: Since a slave owned by a priest or a priests wife can eat terumah, the mixed-up children can eat terumah, for both may eat terumah. When they go to the threshing-floor, the community is responsible to give them only one portion, for only one is a priest. Neither may defile themselves, because each one might be a priest who is prohibited from defiling himself by coming into contact with a dead body. Neither of them can marry any woman because if he is a priest he cannot marry a slave-woman and if he is a slave he cant marry any woman eligible to marry a priest. Since there is no woman who can marry either a priest or a slave, these two men are stuck.
Section two: When they grow up, each may free the other, since one is the owner of the other. At this point, one is a priest and one is a freed slave, but it is unclear who is who. They may now marry any woman they want, as long as she is eligible to marry a priest, since both categories of men may marry any woman eligible to marry a priest. Neither may marry a woman not eligible to marry a priest, for instance a convert, lest he be a priest. Neither may defile himself for the dead, since one is surely a priest. However, if one does defile himself, he is not lashed as is the normal punishment, for it is not certain that he was the priest. A person can only be punished if it was certain that he committed a transgression.
Section three: They are not allowed to eat terumah, since either one might not be a priest. However, if they do eat terumah, they do not have to repay what they ate as do regular Israelites, since each can claim that he is the priest. They do not receive terumah at the threshing-floor, since either one might not be a priest. However, when they separate terumah from their own produce, they may sell it, since other priests cannot prove that the terumah does not rightfully belong to either one of them. In other words, they cannot eat terumah but they need not give it to a priest. They do not receive parts of sacrifices, and they are not given offerings to sacrifice for each is uncertain of his priestly status. However, if they come into possession of sacrifices, other priests may not take them away from them, for each can claim that no one can prove that he is not a priest. When they do bring a sacrifice, other priests may not take the parts of the animal normally given to the priests, for each can claim that he is a priest and no one can prove him wrong. If they have a first-born animal, they do not have to give it immediately to a priest, as is usual. Rather it should graze until it becomes blemished and not valid as a sacrifice. Then they may eat it or make other uses out of it (farming or for wool, etc.).
Section four: This section summarizes the entire mishnah. Basically, the law treats each mixed-up child as stringently as it would a priest (he cant defile himself for the dead) and as stringently as it would an Israelite (he cant eat terumah). However, as with regard to economic issues, other priests cannot make claims from him, since each son can always say to the other priests, prove that I am not a priest. Since there is a general rule that the burden of proof is upon the claimant, he is always exempt.