We have learned that when a ḥatat ha-of – a sin-offering brought from a turtledove or a pigeon – is brought, that the blood of the sacrifice will be placed on the bottom half of the outer altar. The bottom half of the altar is the area under the ḥut ha-sikra – the red line that divided the altar into two parts specifically for this purpose.
The first Mishna of the seventh perek begins on today’s daf, and it opens by teaching that a ḥatat ha-of that was done properly, on the bottom part of the altar with the proper intention, is valid. This stands in contrast with a ḥatat ha-of that is brought in the wrong place (e.g. on the upper part of the altar) or with the wrong intention (e.g. with the intention of bringing it as a burnt-offering), which would be invalid as a sacrifice.
Two things stand out as unusual in the wording of the Mishna. Why is it necessary to state that a ḥatat ha-of that was done properly is valid. Is this not obvious? Furthermore, the ruling that it is “valid” usually denotes that it is valid after the fact. In this case, however, it is not merely valid; this is the ideal way of making this sacrifice!
While Tosafot suggest that this statement is superfluous and comes only by way of introducing other, problematic cases, many of the commentaries explain why this sentence is necessary. According to the Sefat Emet this case teaches about a situation where there is some problem with the sacrifice, for example where the kohen’s intention was for a sin-offering, but for a sin-offering brought from an animal, not from a bird. Even in such a case, the sacrifice would be valid. In his Commentary to the Mishna, the Rambam suggests that this is a case where the melika was not done properly, for example if the kohen did not hold it in the proper fashion (see above, daf 64 for a description of melika).