Meilah, Chapter 2, Mishnah 7. Our chapter has been working its way through the offerings one at a time, marking for each of them when liability for me'ilah begins, when it climbs to a higher grade of kedushah and thereby becomes exposed to certain pesulim, and when me'ilah finally leaves it. The Mishnah now turns to the lechem hapanim, the twelve loaves prepared inside the Mikdash on each Erev Shabbat.
How the lechem hapanim worked
Every Shabbat the twelve loaves were set out upon the shulchan in two piles of six, and beside them were placed the bazichin, the pair of vessels holding levonah. Bread and levonah alike stayed there through the entire week. When the next Shabbat came, the loaves were lifted off the table, the levonah went up in smoke upon the mizbeach, and the kohanim consumed the bread. Our Mishnah traces the successive grades of kedushah that the lechem hapanim takes on as it is prepared and as it is offered.
From the moment the flour is consecrated
The Mishnah opens: "Lechem hapanim, mo'alin bo mishehukdash", one is liable for me'ilah with the lechem hapanim from the moment it is consecrated, that is, from the moment the flour itself is dedicated. At that point the flour carries kedushat damim, a monetary holiness, and it is classified among kodshei kodashim. Deriving benefit from it already constitutes the aveirah of me'ilah.
Once it forms a crust
"Karam batanur", when a crust forms on the dough inside the oven, the loaves reach the stage the Mishnah describes as "huchshar lehipasel": they can now be rendered pasul "bitvul yom uvmichusar kippurim", by the touch of a tevul yom or of a mechusar kippurim. Why only now? Because until the crust appears, the item does not yet answer to the name lechem, bread. Having become what the Torah calls bread, it advances to the next level of kedushah, and that level brings with it exposure to these pesulim.
Notice that the Mishnah does not list lina, disqualification through remaining overnight, as it does elsewhere. That is because the lechem hapanim is baked on Friday and is meant to sit on the shulchan for an entire week until the coming Shabbat. Remaining overnight is precisely what these loaves are supposed to do, so lina could not possibly disqualify them.
The Mishnah continues, "ulehistader al gabei hashulchan": at that same stage, when the dough has turned into genuine bread, the loaves qualify for placement in their rows upon the shulchan.
Once the bazichin are offered
"Karvu habazichin", once the levonah held in the two bazichin has gone up on the mizbeach, the bread becomes permitted food for the kohanim. From that moment, the Mishnah teaches, "chayavin aleha", one bears liability regarding them, "mishum pigul, notar, vetamei": whoever eats the lechem hapanim may incur karet on the count of pigul, of notar, or of tumah.
Pigul. Picture a kohen lifting the bazichin off the table; while he carries the levonah toward the mizbeach, or while he is burning it there, he has in mind that the bread will be consumed beyond its allotted hours. That thought turns the loaves into pigul. Their proper window is the Shabbat on which they are removed from the shulchan, together with the night that follows, precisely the window given to every offering of kodshei kodashim, whose meat is permitted on the day of the shechitah and throughout the night after it. A machashavah reaching past those limits makes the bread pigul, and one who then eats it incurs karet.
Notar. Where the levonah was burned exactly as it should be, and someone eats the loaves once their hours have run out, meaning after that Shabbat together with its night has passed, he incurs karet for consuming notar.
Tamei. And a person who eats from the lechem hapanim while he himself is in a state of tumah is likewise liable to karet.
And me'ilah departs
The Mishnah closes with "ve'ein bahen me'ilah": from the burning of the bazichin onward, the lechem hapanim is no longer subject to me'ilah. It is the levonah that releases the bread for the kohanim's consumption, and once the loaves have become the food of the kohanim they no longer stand as kodshei Hashem in the manner that me'ilah requires. Me'ilah thus withdraws at the very point where pigul, notar and tumah enter.