One of the most common stylistic elements found in the Gemara is that the Gemara first brings a Mishna in its entirety and then quotes short snippets from that Mishna and discusses them at length. Occasionally, after the Gemara quotes a baraita, it chooses to examine the baraita in the manner that it usually analyzes a Mishna, and we find a similar method of short quotes followed by lengthier explanations.
On today’s daf we find that the Gemara brings the following quote: “…and one who slaughters them outside of the Temple courtyard is exempt.” In this case we find disagreement as to the source of this quote, since our Mishna (above, daf 14a) reads that someone who slaughters a blemished animal outside of the Temple courtyard is, in fact, liable.
Rashi suggests that the source of the quote is not the Mishna, but a continuation of the baraita that was brought earlier. He maintains this is referring to animals whose consecration preceded their blemish, and that one is not liable for slaughtering them outside the Temple courtyard because one is only liable for slaughtering an animal that is fit for sacrifice.
From the Rambam it appears that this is a quote from the Mishna and that according to his manuscript of the Mishna even someone who slaughters an animal that became blemished after it was sanctified outside of the Temple courtyard is not liable. According to this reading, the version that appears in our Mishna is a “correction” based on Rav Huna’s teaching in the Gemara. For the Gemara quotes Rav Huna as reading in the Mishna “He is liable.” He explains it as relating to a specific case where the blemished animal was suffering from dukin she-ba’ayin in accordance with the opinion of Rabbi Akiva who permits certain types of mumim to be brought on the altar (or at least that the sacrifice is not removed from the altar if it had already been brought to it).
Dukin she-ba-ayin is some kind of an eye condition; Rashi explains that it is a cataract on the eye. Another possible explanation presented by Rashi is that it is a blemish on the eyelid. Since dukin she-ba-ayin is a relatively minor condition – in fact, it is not considered a blemish if it is found in a sacrifice brought from fowl – Rabbi Akiva rules that an animal with such a blemish is permitted if it found its way to the altar.