The Kiyor must contain water to wash four Kohanim. What is the source for this din? Rambam in Hil. Klei Hamikdash 5:13 brings the pasuk in Ki Sisa, virachatzu mimenu Aharon uvanav, and Banav included Elazar, Itamar and Pinchas. Kesef Mishna brings Rashi Zvachim (19b s.v. Moshe) who brings as the source a pasuk in Pekudei, virachatzu mimenu Moshe v'Aharon uvanav, and banav is 2 people, for a total of 4. What is the reason that Rambam can't use Rashi's pasuk, which seems to make a lot more sense? Usually a plural word is seen as indicating two. Why does the Rambam say banav includes Pinchas, a third person? Also, why does Rashi include Moshe in this law, but Rambam does not?
Avodah Zara 34 says that Moshe served as Kohen Gadol not with the special bigadim, but with a white cloak of some sort. Why didn't he wear bigdei kehuna? Rashi holds because v'asisa bigdei kodesh l'Aharon achicha - it's a pasuk that the begadim are for Aharon, not Moshe. Tosfos (s.v. bameh) asks: This is difficult - according to one opinion in Zvachim, Moshe served as Kohen Gadol for 40 years, and this Gemara should have asked with what garments did Moshe serve for 40 years...? And it appears that they specifically asked about the 7 days of Miluim because the bigdei kehuna were not yet sanctified as the pasuk says 'you will sprinkle [blood] on Ahron and on his garments [indicating this process also sanctified the garments].' And R' Yaakov m'Orleans answered that during 7 yemai miluim, the Mishkan had a din Bama (large communal altar whose use was banned when there was a mishkan or Beis Hamikdash), and there are no bigdei kehuna by a bama.
I would like to suggest that Rashi held Moshe's service during 7 yemai miluim was a full service of kehuna gedola, so the capacity of water in kiyor can be learned from Moshe's service during the seven days of miluim. Rambam, however, held like R Yaakov m'Orleans that 7 days of miluim were not like the mishkan, they were Bama. So Moshe did not serve as kohen gadol in the mishkan, so Rambam couldn't learn the minimum volume of the Kiyor from Moshe during the seven days of miluim, and had to use a pasuk that did not include Moshe, and he had to mention that Pinchas was one of the Kohanim.
With this in mind, we can understand another machlokes between Rashi and Rambam. Rambam says (Hil. Klei Hamikdosh 2:6) that the annointing oil must be made in a "kli shareis," a vessel of the mishkan/mikdosh. If so, there is a question: the vessels in the time of Moshe were sanctified by being annointed, but how could they have been annointed, because there was no sanctified vessels in which to make the annointing oil? This is the question of the Brisker Rav (see Stencils 110, Mishor ed.) who did not offer an answer. But the answer is found in Meshech Chochma (Shemos 39:33): Moshe made the annointing oil during the seven days of miluim before there was a status of mishkan and kli shareis. When the status of Mikdosh did not exist, you could make the annointing oil in a non-Mikdash vessel. This idea is in accordance with our understanding of Rambam's opinion. However, the question remains unresolved according to Rashi. Therefore the Meshech Chochma says Rashi is forced to agree with the opinion of Reish Lakish in Yerushalmi Shekalim 4:3 that if one made the annointing oil in a non-Mikdash vessel, it is acceptable.
For a related post, see here.