The gemara (Avodah Zarah 5b) derives from the pesukim describing the animals which Noach took on the ark that a ben Noach is prohibited from sacrificing an animal which is missing a limb. Tosfos asks why this should not count as an additional mitzvah in the count of the mitzvos applying to a bnei Noach, i.e. there should be 8 mitzvos bnei Noach, not 7. Tosfos answers that the 7 mitzvos are a count of issurim. The halacha of not offering an animal with a missing limb is a mitzvah is b’kum v’aseh – if a ben Noach wishes to donate a korban, we demand that he fulfill his pledge with a complete animal and not one missing limbs. R’ Akiva Eiger asks how this can be so. If a Jew pledges a korban, his words bind him to bring a kosher korban because of the issur of “bal yachel” and the mitzvah of “motza sefasecha tishmor”. However, these mitzvos do not apply to a ben Noach. If a ben Noach pledges a korban, what indeed is the mitzvah that would compel him b’kum v’aseh to fulfill his pledge?
With respect to Tosfos’ question (which I mentioned over lunch on Shabbos), my wife gets credit for the ingenious observation that the 7 mitzvos bnei Noach are issurei gavra on the person. The issur of offering a missing limb may be an issur cheftza which blocks the korban from being offered and hence is placed in a different category.