Over Pesach a very bright bachur asked me a kashe on the din that a ger who converts before Pesach has the din of "poreish min hakever", i.e. it is as if he just became tahor from tumas meis and therefore even after "milah and tevilah" he must wait 7 days until bringing his korban pesach (Rambam K.P. 6:7). Why does the Rambam say only "milah and tevilah" and not mention the korban which a ger is obligated to bring? I did not (and still do not) have a good answer.
While on the topic, the Rambam quite unusually presents a little shakla-v'tarya in this halacha. He asks - how could chazal have uprooted korban pesach, which is a chiyuv kareis, with a mere gezeirah derabbanan? Rambam answers that chazal instituted that the ger should not be toveil until after 7 days; since the geirus in incomplete until that point, the chiyuv korban/kareis never sets in. Why did the Rambam not simply answer "yesh koach b'yad chachamim" to uproot a mitzvah b'shev v'al ta'aseh? It would seem from the Rambam that this rule is inapplicable when kareis in involved. Rishonim (e.g. the Meiri) and achronim alike ask why the gezeirah d'rabbah that teaches us not to blow shofar or take a lulav on shabbos lest it be carried in a pubic domain not also apply to cancel a bris milah on shabbos. According to the Rambam, this would not be a kasha, as bris milah is an issur kareis and hence cannot be circumscribed by a gezeirah.
My question for you sharpshooters: the gemara in Shabbos 3b asks whether Chazal prohibited "rediyas hapas" (scraping bread dough out of the oven), an issur derabbanan, if the bread in the oven is about to become cooked in violation of an issur d'oraysa of ofeh. Rashi writes "...rediyas hapas shvus hu v'nigzerah b'minyan, u'mshum shema kodem she-yavo l'yedei chiyuv chatas lo bitlu gezeirasam" - rediyas hapas was a formally enacted gezeirah, and even to avoid the possibility of chatas, the chachamim did not restrict the gezeirah. In the Rambam's case, enforcing a gezeirah derabbanan potentially causes an issur d'oraysa of bittul korban pesach; here enforcing the gezeirah derabbanan of rediyas hapas potentially causes an issur d'oraysa of ofeh. Is this the same issue (albeit anissur Shabbos instead of pesach), or would you be mechaleik? How?