Negaim, Chapter 13, Mishnah 6. This mishnah, which closes the perek, deals with a person who acted against the halachah: he removed stones from a house that was under quarantine and built them into a house that was perfectly tahor. The mishnah traces what happens to both houses and to the stones themselves once the nega runs its course.
The case
The mishnah opens: "Haboneh min hamusgar batahor," one who builds from the quarantined house into the clean house.
The Tiferes Yisroel, and the Rav along with him, fill in the background. We are not standing at the beginning of this nega's history. A first week of hesger has already gone by, the color held its ground, and then it spread. A pisyon at the end of that first week obligates the kohen in chalitzah (prying out the afflicted stones), ketzi'ah (scraping the plaster around the spot) and ticha (setting in replacement stones and resurfacing with fresh mortar). Netitzah, tearing the building down, is reserved for a nega that holds on through three weeks. So our mishnah says "l'achar shecholetz v'kitzah v'tach v'nasan lo shavua": the extraction, the scraping and the replastering are behind us, and the kohen has assigned the house one more week of observation. Somewhere in the middle of that week, whether it is the second or the third, a man comes along and pulls stones out of this house in defiance of the halachah, "uveno'an b'bayis acher", fixing them into the wall of a different house, one that is entirely clean.
When the nega returns to the first house
"V'chazar nega labayis, choletz es ha'avanim." The nega returned to the original house. A return of the nega after chalitzah, ketzi'ah and ticha means that the house is condemned to netitzah. And the stones that were carried off are treated as though they had never left: they carry exactly the same tumah as if they were still set in the wall of the first house. Therefore he must extract those stones from the clean house into which he built them, even though no nega ever appeared on them, and they too are subject to netitzah along with the house from which they came.
When the nega appears on the transplanted stones
"Chazru la'avanim, habayis harishon yinatetz, veha'avanim yeshamshu es habayis hasheini b'simanim." Now the reverse: the nega did not reappear in the original house but broke out on the very stones that had been relocated into the tahor house. The first house is nonetheless demolished, since these stones are regarded as though they were still part of it, and a return of the nega after chalitzah, ketzi'ah and ticha demands netitzah. As for the stones themselves, they now "serve the second house with their signs."
The Rav lays out the reasoning. The quarantined house comes down because the halachah looks at the relocated stones as though they had never been pried out of its walls. Those stones are themselves headed for destruction; the only question is when. We hold off on them so that their simanim can first run their course in the building where they are now lodged, for that second house has become a house with a nega on it and must be shut up. In every particular it is judged exactly like any house in which a plague broke out for the very first time.
The second house begins its own process
The second house now enters the standard track from the beginning. It is shut up for a week. If nothing develops, if the nega fades or vanishes, the house itself is spared and remains tahor. If after the first week there is a pisyon, the kohen performs chalitzah, ketzi'ah and ticha, removing the offending stones (whether these very stones or others in which the spread appeared) and replastering, and then one waits to see what follows. And after ketzi'ah, any chazarah, any return of the nega, condemns that house to demolition as well.
But whatever the fate of the second house, the transplanted stones do not escape. Even if the house is ultimately declared tahor, those stones must be taken out and destroyed, exactly as if they had remained in the first house. They are merely allowed to stay in place long enough for their simanim to determine the status of the house they were built into.
The parallel in nig'ei begadim
There is a mishnah built on precisely the same pattern in Perek Yud-Alef, in the laws of nig'ei begadim, and our mishnah makes its meaning much clearer. There the mishnah teaches "hatoleh min hamusgar batahor": a person took a patch from a garment that was under quarantine and sewed it onto a garment that was tahor. If the nega returns to the original garment, "soref es hamatlis," the patch must be taken out of the clean garment and burned together with the garment from which it came. If the nega appears on the patch itself, "habeged harishon yisaref," the first garment is burned as though the patch were still attached to it, exactly as the first house here is demolished, "v'hamatlis teshamesh es habeged hasheini b'simanim," and the patch remains to serve the second garment with its signs before it too is burned.
The Rav writes there along the same lines. The quarantined garment is burned wherever it happens to be found, since the halachah regards the transferred piece as if it were still attached to it. That piece, in the meantime, works with its simanim on behalf of the garment into which it has been sewn, and the second garment is treated as though the nega had surfaced on it directly: it is quarantined like any afflicted garment, it can be consigned to the fire like any afflicted garment, it is washed like any afflicted garment, in every respect without distinction. Then comes the line that settles the whole matter: once the patch has completed its service, it must be burned even if the nega has vanished from it entirely.
Picture it: after that first week there was a pisyon elsewhere in the second garment, while the original patch became completely clean; or alternatively nothing at all happened, the garment was set aside for the week of hesger, no change appeared, and the second garment was pronounced tahor. In every one of these cases the patch has already become huzkak l'sereifah, already bound over to burning, and that status does not lapse. Once the second garment's own status has been settled through hesger, the patch is removed and burned, precisely like the garment from which it was taken.
The two mishnayos are one law in two forms. Just as the stones here are destined for netitzah and merely remain in place until the fate of the clean house has been determined, and are then removed and destroyed whichever way that determination goes, so too there: whether the second garment turns out tahor or tamei after its hesger, the matlis is taken out and burned.