The Mishna on our daf teaches about situations where newborn children are inadvertently switched and we do not know which child belongs to which mother. For example, if the child of a kohen was mixed up with the child of his wife’s maidservant, the Mishna teaches that both children can eat teruma (either as the child of the kohen or as the property of the kohen), and neither can come into contact with a dead body (since we cannot know which one is really the kohen and which one is not.)
In clarifying the rules of a kohen and his servants, the Gemara relates a disagreement between Rabbi Yehuda and Rabbi Yose. Rabbi Yehuda insists that such a person can only receive teruma in the granary if he is accompanying his master, the kohen. Rabbi Yose allows the servant to receive teruma on his own, with the argument that he is deserving of the teruma, either on his own merit as a kohen, or else because he is owned by a kohen. The Gemara explains that this disagreement is based on different practices that existed in each of their communities. In Rabbi Yehuda’s community, receiving teruma was considered tantamount to proof that the recipient was a kohen; in Rabbi Yose’s community, it was not considered proof.
Rabbi Elazar bar Tzadok then relates that this difference almost led to a servant being accepted as a kohen. As explained by the Gemara, while in Rabbi Yose’s community he saw someone receiving teruma. Upon testifying to this fact in Rabbi Yehuda’s community, the man was almost accepted as a kohen.
The Gemara is certain that ultimately no mistake had been made, arguing that “if no error can ensue from the activities of the animal of a tzaddik, certainly the actions of an actual tzaddik cannot lead to error.” The source for this concept is a story that appears in Hullin (7a) where Pinehas ben Ya’ir’s donkey refuses to eat food that had not been properly tithed. Although Rabbeinu Tam also extends this idea to other areas of halakha (as in our case), Tosafot on our page erase the discussion of this idea, arguing that it only applies to eating forbidden food, and not to other possible errors.