From: Rabbi Akiva Eiger
To: Unknown recipient
Rabbi Akiva Eiger raises an unresolved difficulty: the Gemara in Makkos classifies a party to an act as a ba'al din rather than a witness, yet the Mishnah in Sanhedrin explicitly designates the nisas (one incited to idolatry) as a witness. This creates a problem since the Torah envisions a mesis who is a relative of the nisas, which should disqualify the nisas as a witness and void the entire testimony under the rule of nimtza echad meihen karov o pasul. No resolution is offered.
Source: Shu"t Rabbi Akiva Eiger §211