Dueling Chumras

Note: Not halacha lemaaseh, or intended as such. About anything like this, consult your local Orthodox rabbi.

Sometimes the only thing that stops the spread of a chumra is the bounds set by another chumra. This would be because while people are willing to roll over previously acceptable lenient practice, when someone raises the possibility that they are being lenient regarding another din, or another chumra.

I think we can cast as one such example how the stringency of Pesach cleaning up of every tiny crumb much less than a kezayis duels with the requirement not to eat matza on erev Pesach.

The obligation to do away with any chametz -- bal yeraeh / bal yimatzei really applies only to chametz equal to or more than a kezayit. This based on various gemaras which talk about dough in a vessel, or on a wall, and whether it combines to a kezayit which then must be gotten rid of. Indeed, Tur says that based on this, it is only chal on >= a kezayit.
(Other acharonim might argue, but this seems to be correct. And indeed, this seems pashut peshat in the gemara in Pesachim 6b which states
אמר רב הבודק צריך שיבטל מאי טעמא אי נימא משום פירורין הא לא חשיבי
Crumbs which are not chashivei need not be worried about.
)
But Tur notes that the Jews are holy and their accepted practice is to eliminate any chametz, no matter what amount.

There is a separate requirement not to eat chametz, and there, it is even bemashehu, any amount. These two defined requirements are counterintuitive since we would expect the issur achila to be the one with the kezayit and the other with the other shiur, and so people even today confuse them -- which may have led, IMHO, to the stringency of removing every crumb

Because of this adopted stringency, people take a lot of time getting rid of every single crumb from every crevice, and so Pesach cleaning takes much longer than it otherwise might have taken. And once all this effort has been expended, we don't want to bring chametz back into that room, for it is already kosher for Pesach, and we do not want to expend all that effort again.

I remember a year or two back, it was close to Pesach, that is that Pesach was a few days away, and I was by people who did not want to have the meal at home because it would involve dealing with chametz. They discussed buying pizza. But they would not bring the pizza into the home, to be eaten by the table. I noted that they could eat the pizza and throw out any remainders. But they were afraid of crumbs. So a sweep of the linoleum floor would suffice. But they might miss a crumb or two, and did a real thorough job (perhaps mopping, and so on), so this was not feasible. Instead, they ate out at the pizza store. So this is a stringency even in cleaning -- not only will you sweep to get rid of the crumbs which one is not required to get out, but one must be extremely careful in this elimination lest one crumb escape. And rather than doing this again and again, or rather because this effort must be done gradually over time, to maintain this non-required level of absence of chametz, no chametz may enter the house at all.

The pasuk, meanwhile, says that 7 days leaven shall not be found in your house. Not 3 weeks.

I know people who are faced with a dilemma when erev Pesach falls out on Shabbos. You can't eat matza because of another prohibition (to be dealt with in a moment). So it must be challah. But disposal is a problem. So they get a tiny amount of challah, enough to fulfill the obligation, and eat it on the porch. {Update: In fact, to clarify, they do this any Shabbos before Pesach, even if it does not fall on erev Pesach.}

If this was unchecked, I am pretty confident that Jewish practice would have evolved to remove the consumption of chametz, at least in the house, a few weeks before Pesach. For Shabbos, all we need to do is eat matza!

But there is another halachic statement, which expanded into the realm of chumra itself. The gemara states that kol h`aochel matza erev Pesach ke`ilu bo'eil arusaso beveis chamav. (Jackie Mason Achad HaAm Bialik, BTW, has an interesting line about this: "I tried both, and it's not the same.") It is not brought down by the Mechaber but it is by the Rema who prohibits the entirety of the 14th. The Mishneh Berurah notes the minhag not to eat matza from Rosh Chodesh. And elsewhere (unsourced), the minhag has developed not to eat it from Purim. (We might contrast this with the statement shebechol haleilos anu ochelin chametz umatza which might suggest that every night on which we can eat, which is not Pesach, we can eat either chametz or matza.)

So this extension of the halachic requirement blocks any potential imposition of eating (only) matza in the weeks before Pesach. :) Which is a good thing.