From: Rabbi Moshe Sofer
To: Rabbi Eliyahu Neiman
When two witnesses testified that the shochet's knife was defective after slaughter and two others contradicted them, the resulting stalemate (trei u'trei) creates a rabbinic-level doubt sufficient to prohibit all meat and vessels from the day of testimony onward. The shochet's current inability to feel blade defects does not retroactively prove past incompetence, since gradual sensory decline from aging is natural and does not tip the scales in favor of either set of witnesses.
Source: Shu"t Chasam Sofer YD §2