Nazir 52 - Skull and spine
If a nazir comes in touch with a dead body, he loses his previous days, has to shave, must bring sacrifices, and then restart. How much of a dead body? One of the example is a skull and spine - just bones, even if they have no meat on them.
There were actually two versions of this rule, one read "skull and spine," but the other one - "skull or spine." According to the second one, either a skull or a spine are enough for nazir to loose his days and shave. Which one is right?
The Talmud tries to resolve this in many ways. For example, since there are six questions related to nazir where Rabbi Akiva initially held a more stringent position but eventually conceded to the Sages, if his position was more stringent here too and read "skull or spine" - then the total count would be seven, not six. However, Rabbi Shimon says that there was another rule, about a "quarter-measurement of blood from two different dead" where Rabbi Akiva never conceded. This changes the count and ruins our proof.
Incidentally, Rabbi Shimon emphasized his points as follows, "While Rabbi Akiva was alive, he never changed his mind on this question, and whether he retracted after death - I don't know about that." Unfortunately, this was not the most refined way of putting it. Rabbi Shimon felt remorse and fasted, to atone for it, until his teeth darkened.
Art: An old man, holding a skull by Jan Lievens