We've been saying that a Bechor (firstborn animal) that has a blemish is allowed to be slaughtered and eaten right away. But that's only talking about a blemish that is a mum kavua, a permanent blemish. A temporary blemish that may go away is different: you're not allowed to do Shechita (slaughter), because it may heal. It may heal, and the Bechor may not be blemished at all. So before a person does the Shechita, he has to show the blemish to a mumcheh, to a Hacham, an expert at evaluating these blemishes, to determine whether the blemish is permanent or temporary.
That expert has to be an individual who understands animal blemishes very well and has permission from the Nasi (president) or the Beit Din (court) in Eretz Yisrael before he examines these blemishes. We'll talk here about a blemished Bechor that was wrongfully slaughtered before it was examined by a mumcheh.
The Mishnah says: If someone slaughters a blemished Bechor and only afterwards shows its blemish to an expert, a Hacham, who determines afterward that it is a permanent blemish, but at the time of Shechita they didn't know anything about it, Rabbi Yehuda matir. Rabbi Yehuda says, “The Bechor is permitted to be eaten.” The blemish was found to be permanent afterward. Okay, they should have checked it beforehand, but now that it's checked afterward, we could retroactively permit the meat and say that the Shechita was done in a halachically okay way.
Rabbi Meir omer: Since the Bechor was slaughtered improperly without the approval of a mumcheh, an expert, it's forbidden to benefit from the Bechor. It has to be buried. It's like a Bechor that dies without being properly slaughtered. If you don't have that stamp of approval from the mumcheh, things are not okay. He would hold as well that the whole idea behind this is that blemishes could change. Death, the pain of death, could change how certain things look, and therefore you don't have a good look at things post-death.