As we learned at the beginning of the eighth perek (=chapter) of Masechet Zevahim, the various responsibilities of the kohanim who were working in the Temple would invariably lead to mistakes and confusion on occasion (see above daf, or page, 70). Having completed the examination of the laws regarding sacrifices that became mixed up with one another, the ninth perek (=chapter) of Masechet Zevahim, which begins on today’s daf, focuses on a different question – what should be done with invalid sacrifices that somehow make their way to the altar.
From the passage in Sefer Shemot (29:37) that teaches that “anything that touches the altar will become holy” the Sages derive that animals that are appropriate for sacrifice will become fully sanctified if they are brought onto the altar, even if there is a problem that would, ordinarily, cause them to be invalid for sacrifice. In the language of the Mishnah, keivan she-alu, shuv lo yerdu – once they were elevated onto the altar, they cannot be brought down, and they will be sacrificed. There are two somewhat surprising elements in this ruling – first that the invalid sacrifice is not removed from the altar, and secondly that it is treated as an ordinary sacrifice once it is on the altar.
The first Mishnah in the perek makes clear that this law does not apply to anything that is brought to the altar, only to animals that are appropriate for sacrifice. The source for this idea is the passage in Sefer Vayikra (6:2) that teaches that the olah – the burnt-offering – is brought on the altar, and the Sages derive that just as the olah on the altar can only be an animal that is worthy of sacrifice, so, too, all other cases of sacrifice are limited to animals that are worthy of sacrifice.