If the shechitah did not allow for the meat of the animal to be eaten, then Rabbi Shimon does not consider this act as shechitah. Therefore, if one made a shechitah but found that the animal was a terefah anyway, or if he slaughtered an idolatrous sacrifice, or a bull sentenced to be stoned for goring, a red heifer, or a calf designated to be decapitated to atone for a unresolved murder case, and then slaughtered its offspring, then according to Rabbi Shimon he is not liable for slaughtering an animal and its offspring on the same day.
In fact, according to Rabbi Shimon, the prohibition of “it and its offspring” does not apply to sacrifices at all. Rav Hamnuna said, this is because after slaughter the meat is not ready to be eaten until its blood is thrown on the Altar, and thus it is not a shechitah. Rava said that this is because perhaps they will not throw the blood on the Altar. Then the warning they can give him becomes doubtful at best, and one is not liable for transgressing a law without a clear unequivocal warning immediately prior to the act.
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