Zevahim 115: Before the Kohanim... There Were the First-Borns

A dispute over whether one is liable for slaughtering a premature guilt-offering outside of the Temple - a very specific case. Also, if what you slaughter isn't fit to be a Temple offering to begin with, for example, than there's no liability. Plus, offerings that were allowed to be brought outside of the courtyard (or, rather, the Tabernacle) because it hadn't yet been established as the sole location for this. Thus, in the wilderness - offerings were made in the Tent of the Meeting, and largely by the class of the first born, rather than the kohanim. At least, until the kohanim were established at the day of establishing the Tabernacle itself. Also, the deaths of Nadav and Avihu, their father's reaction, their uncle's reaction, and what it means to sanctify God in their deaths.