Shevuot 14: Hidden Impurity
The end of chapter 1! Does the goat that is sent to "Azazel" atone for kohanim? Unclear, but they have other means of atonement. Plus, the dispute between Rabbi Yehudah and Rabbi Shimon on atonement. And, with the new chapter, a long mishnah, beginning with the case of a person who touches an impure thing and then enters the holy (or handles the holy foods) - but inadvertently (namely, the transgression is "hidden from him" - the consequences depend on the particulars. And the mishnah continues with many cases, with details about purity -- including extending the size of Jerusalem or the courtyard of the Temple. Plus, the case of where the action is known, but not the impurity (specifically in the case of a sheretz -- creepy-crawly).