From: Rabbi Moshe Sofer
To: Unknown recipient
The Chasam Sofer analyzes the rule of nimtza echad mehem karov o pasul — when a disqualified witness is found among a group, whether the remaining qualified witnesses' testimony is also invalidated. He resolves Tosfos' challenges against Rashi by distinguishing between the stage of observation (re'iyah) and the stage of testimony (hagadah), arguing the disqualification of joint observation requires the special verse v'hitzilu ha'eidah, which applies only in capital cases. He further explores implications including the status of retroactively disqualified witnesses and the combining of remaining witnesses with new independent witnesses.
Source: Shu"t Chasam Sofer CM §13