The Rambam describes that the korban Pesach must be broiled in order to remove the blood which is found in the animal's capillaries. Additionally, the larger arteries and veins must be removed before the korban is broiled, to avoid the blood they contain being absorbed into the flesh and rendering it treif. After the korban is broiled, the gid hanosheh, which may not be eaten, is removed. The Raavad, in his satirical style, writes, "if I would be in the Beis Hamikdash when this man (the Rambam) brought his korban, I would throw it on the floor and tell him that he is not permitted to eat it because the gid hanosheh makes all of the flesh of the korban treif." Rav Yosef Caro (in his commentary Kesef Mishneh) comes to the defense of the Rambam and explains his position, as follows: the accepted view in the Gemorah (Chulin 99) is that the gid hanosheh does not have enough taste to make the rest of the korban treif. Normally we would certainly recommend removing the gid hanosheh before cooking or broiling meat with it; but in the case of the korban Pesach there is a special din that one is supposed to broil the entire animal intact and slice it up after the broiling. Since b'dieved the gid ha'nosheh does not make other food treif, we leave it in when we broil the korban Pesach in order to fulfill the mitzvah of broiling the korban when the entire animal intact. Regarding the veins and the arteries, however, we have no choice but to remove them, since having them in place when the korban is broiled would render the entire korban treif.
Rav Yosef Caro raises an objection to this defense. The gemorah tells us that although the fat which is on top of the gid hanosheh is biblically permitted, "the holy Jewish people have the practice of not eating it". As such, one cannot say that the fats on top of the gid hanosheh don't have any flavor, and therefore if one broils the entire korban Pesach when it is all intact the flavor of these fats will spread into the flesh and everything will become treif. Rav Yosef Caro himself responds that from the gemorah it would appear the even the fat on top of the gid hanosheh is permissible not only biblically but even m'dirabbonon, and the practice of not eating this fat is only a minhag. In this halacha the Rambam is teaching us that whenever there is a conflict between a real din and a minhag, we must observe the real din. Therefore, once a year on seder night, all the Jews in Jerusalem would be violating the minhag of not eating the shuman ha'gid because removing this fat would be a violation of the din of broiling the korban Pesach when it is all intact.
Rav Yisroel Gustman was a great goan who survived the Nazi Holocaust. At the end of his sefer on Kiddushin, which is full of chiddushim on the gemorah, he included a teshuva halacha l'maaseh on a contemporary issue, the thrust of which is that, as Rav Yosef Caro pointed out, whenever a minhag is in conflict with the real halacha, the halacha certainly takes precedence.