Why by the Chata'os of all the Yamim-Tovim, does the Torah add the (otherwise superfluous) word "Le'chaper aleichem" except for those of Sukos?
Why does the Torah not say that the goat on Shavu'os is a Chatas like it does by all other Musafim (except for Shabbos, which has no Chatas)?
Ba'al ha'Turim (on Pasuk 27): Because the Torah was given on Shavu'os, and the Torah atones for those who study it ? in which case it does not require a Chatas.
Da'as Zekenim (on Pasuk 25) #1 and Hadar Zekenim: On account of Kavod ha'Torah, the Torah does not mention Chatas on the day on which the Torah was given.
Da'as Zekenim (on Pasuk 25) #2, Moshav Zekenim nd Hadar Zekenim (citing the Yerushalmi Rosh Hashanah, 4:5): From when Yisrael accepted the yoke of Torah and Mitzvos, Hashem considered it as if they had never sinned. 1
As the Piyut states 'Accept My kingship, and you will not have guilt!'
How do we know that the "Se'ir Izim" is is not the goat that is brought together with the Sh'tei ha'Lechem?
Rashi (in Vayikra): Refer to 23:19:2:1. Because the Olah is different (for Musaf there are two bulls and one ram, and with the bread are one bull and two rams, so surely also the Chatas is different).
Da'as Zekenim: Because whereas by the bread, the Torah calls the goat a Chatas, here it does not.