From: Rabbi Moshe Sofer
To: Rabbi Eliyahu Moshe Katz
A shochet confessed on his deathbed that he had repeatedly slaughtered with a blemished knife due to social pressure over four years, raising questions about distributed fat and contaminated utensils. The Chasam Sofer rules leniently, holding that since the only evidence derives from the shochet's own confession, the principle of peh she'asar hu hapeh she'hitir applies, and since the majority of his slaughter was valid, the fat and utensils are permitted under kol d'parish m'ruba parish.
Source: Shu"t Chasam Sofer YD §22