If a man slaughtered an ordinary animal outside the Temple precincts, declaring it to be a burnt offering or a peace offering or a guilt offering for a doubtful sin or the Passover offering or a thanks offering, the slaughtering is invalid; Rabbi Shimon, however, declares it valid.
According to the letter of the law, someone who makes such a statement has not accomplished anything, since this animal had not been set aside as a sacrifice, and it is unusual to consecrate it in this manner. Nevertheless, the Sages ruled that the slaughter should be considered invalid since it appears as if he is slaughtering consecrated animals outside of the Temple, and people who see this may mistakenly think that that is permitted. Rashi explains that it is for this reason that the Rabbinic enactment forbidding such slaughter only applies to those sacrifices that a person donates and not those that are obligatory. Rashi explains the continuation of the Gemara, as well, based on the approach of marit ha’ayin– the perception (or misperception) that this action will engender in others.
There are other rishonim, however, who explain that this Mishna refers specifically to the historical period when the Temple stood, and that the concern expressed in the Mishna relates to the fact that the man’s statements may actually confer sanctification on the animal.
The Ba’al haTurim suggests that based on this reasoning even today when the Temple is not standing we must be concerned with the possibility that the animal becomes consecrated by means of his statement and that he is effectively performing sheḥita on kodashim ba-ḥutz – a consecrated animal outside of the Temple. At the same time others point out that this cannot literally be a case of kodashim ba-ḥutz since the animal is considered non-kosher, but it is not forbidden to derive benefit from it.