As we learned in the Mishna (daf 15), the second perek of Massekhet Zevaḥim deals with situations where there is some problem with the person who brings the sacrifice. One of the examples that appear in the Mishna is the case of yoshev – when the kohen is sitting at the time that the sacrifice is brought.
What is wrong with sitting while performing sacrificial service?
Rava quotes Rav Naḥman as saying that this is based on the passage (Devarim 18:5) that teaches that the kohanim were chosen by God to “stand and serve” in His name. We can conclude that only when they are standing are they chosen to serve as priests.
The Gemara quotes a baraita that suggests that this pasuk would only serve as a positive commandment; the requirement to do so that teaches that the sacrifice will be invalid if the kohen was not standing stems from the continuation of that discussion where the need to stand is repeated (see Devarim 18:7).
The Gemara records that Rava asked Rav Naḥman why someone who performs the sacrificial service sitting would not be liable to receive a death penalty, given that based on the pasuk such a person was not chosen to serve in the Temple, and should be considered to be a zar – a non-kohen – whose punishment for such an act would be death. Rav Naḥman’s response is that in cases where the Torah condemns people to death for such infractions (e.g. a kohen who serves without the proper priestly clothing) it states the punishment clearly, and we cannot derive a similar punishment for this case.
While Rashi appears to understand this discussion according to its simple meaning – that there is an assumption that someone who is sitting should literally be considered to be a non-kohen, and liable to a death penalty – other rishonim reject that possibility. They suggest that it is clear that such activity would only affect the sacrificial service, but not the kohen himself, and that Rava’s question was whether we could derive the appropriateness of a death penalty from similar cases, like that of a kohen who serves without the proper priestly clothing (see above, daf 17).