Chapter 13! On offering sacrifices in locations that are not specified, and therefore also prohibited for sacrifices. The new mishnah raises the cases of offering outside the Temple. With a difference of approach between Rabbi Yossi HaGalilee and the sages. Plus, a ritually pure person who ate that which was impure would be exempt from a sin-offering, and the mishnah provides the explanation why. Also, the sources for all this in verses. Also, a focus on why this kind of offering outside of the Mishkan (or Temple) can't carry a "karet" punishment as derived from logic - though the Gemara first attempts to show how that logic would be upheld (by Rabbi Avin - and then knocked down by Rava).