Intentional national sin is too explicit in our history to deny. But some try to deny the possibility of Shogeg, at least. This way, they are saved from ever needing to re-examine things like Heter Iska and Mechiras Chametz (assuming these heteirim are, in fact, innocent). The parsha of special korbanos for national errors is just ignored (it’s probably like Ben Sorer which some say never happened and never will).
The proof is adduced from Iggros Moshe Yoreh Deah volume two, at the beginning of chapter 146. The topic is germs not qualifying as sheratzim (insects we are forbidden to consume). Starting with not paskening halacha using a microscope (whose source he quotes as Rabbi Chaim Soloveichik*) for “Pegima” or “Mar’os” שג”כ הוא דין ברור ופשוט שאין להסתפק כלל, Rabbi Feinstein then adds the following regarding sheratzim :
ואף אנחנו קטני קטנים, ידענו לדבר פשוט שא”צ לפנים כי לא הוזכר בגמ’ וכל הדורות הכשרים הגאונים והצדיקים לא השתמשו במיקראקאף, וברור שהם קיימו כל התורה ולא נכשלו בשום דבר אף באונס.
Of course, one can find examples of all Jews doing aveiros in Onness, such as the Yerushalmi of the dead body buried under the Mizbe’ach. But that is not the logic here. He is also not saying Chazal’s scientific knowledge is inevitably infallible and unchanging. He is not even saying that Hashem’s intention of having mitzvos performed would have been thwarted had they not been due to no fault of their own. All of these claims are arguable. His actual point is that it is implied or explicit in Divinely inspired Torah works, such as Scripture (כן עשו בני ישראל) or Shas that the Torah was frequently kept, and it must be the case the Torah was possible to be kept from the very moment it was granted (even if improvements are possible because of historical and technological changes ).
(By the way, see the continuation of the Teshuvah for how the questioner was willing to kill people he technically knew to be alive.)
Here is the part of Iggros Moshe quoted above, from HebrewBooks:
* There is a story of Rabbi Soloveichik using a magnifying glass for examining an Esrog, when he was corrected by a young Chazon Ish, so perhaps the Chazon Ish is the true source.