From: Rabbi Moshe Sofer
To: Rabbi Hirsch Kunitz
The Chasam Sofer rejects two leniencies proposed regarding a mouse found in solidified fat: that the mouse's presumed living status offers relief (since a live sheretz emits taste just as a dead one does), and that we may assume it just fell in (since the principle of machazkinan issur mizman l'zman establishes prohibited status retroactively). He also critiques the Pelesi's reading of Tosfos, arguing that retroactive presumption applies universally in time-based cases, not only in kodshim.
Source: Shu"t Chasam Sofer YD §94