Gemara

78a (Rava): If witnesses testified about a Treifah and they were Huzmu, they are exempt. If Treifah witnesses were Huzmu, they are killed.

Rebuttal (Rav Ashi): Also Treifah witnesses who were Huzmu are not killed, for the testimony of the Mezimim (who said that the witnesses were elsewhere) cannot be Huzam (we could not kill them for trying to kill Treifos).

85a (Beraisa): If one was being taken to be executed, and someone other than his son hit or cursed him, he is exempt.

(Rav Mari): "B'Amcha" refers to one who will remain in your nation (it excludes someone sentenced to die).

Bava Kama 75b (Beraisa): If two witnesses testified that Reuven stole (a cow or sheep), and two others testified that he sold, and the latter witnesses were Huzmu, Reuven pays Kefel, and the Edim Zomemim pay two or three;

Sumchus says, the Edim Zomemim pay Kefel, and Reuven pays two or three.

(Rav Acha brei d'Rav Ika): The case is, two witnesses said 'you stole', and Reuven answered, 'I did not steal in front of you, rather, in front of Ploni and Almoni, and Ploni and Almoni testified that he stole and sold. Chachamim hold that since Ploni and Almoni cannot be Huzmu (for Reuven admitted that they witnessed it), their testimony is invalid. Sumchus says that even so, their testimony is valid.

Makos 6a - Question (Rav Papa): According to R. Akiva, who says that if one of three witnesses was found to be a relative or Pasul, the testimony is invalid, a murder victim should disqualify all the witnesses!

Rishonim

Rambam (Hilchos Edus 20:7): If witnesses testified that a Tereifah (Levi) murdered and they were Huzmu, they are not killed. Even had they physically killed Levi, they would not be killed, for he is a Tereifah. Similarly, if Tereifah witnesses testified about a capital case and they were Huzmu, they are not killed, for if the witnesses who were Mezim them would be Huzmu, they could not be killed, for they were Mezim Tereifos.

Kesef Mishneh: How can we judge according to Tereifah witnesses? It is Iy Efshar Lehazimah, for we could not kill witnesses who are Mezim Tereifos! We can say that when they testified, we did not know that they are Tereifos.

Ohr Some'ach: One can be Mezim Tereifah witnesses for monetary cases. If they are Pesulim for capital cases, why wasn't it taught that they are Kesherim only for monetary cases? Sanhedrin 85a connotes that one is not lashed for hitting a Tereifah. If so, we cannot lash Mezimim whose testimony would have obligated Tereifos to be lashed.

Ohr Some'ach (Hilchos Edus 3:5): Even if testimony of Mezimim is Iy Efshar Lehazimah, it suffices to exempt the Nidon. It is no worse than contradiction, which exempts the Nidon, even though we cannot obligate the latter witnesses who contradict the first witnesses. Why does Rava say that Edim Zomemim is a Chidush, and even when the Mezimim cannot be killed (e.g. the witnesses are Tereifos), we kill the witnesses? We should exempt the witnesses, and therefore, also the Nidon! Indeed, Rava exempts him, for the testimony cannot be Huzam. Rava discussed healthy witnesses whose testimony was accepted, and afterwards became Tereifos. Now, Hazamah of them is Iy Efshar Lehazimah, for we cannot kill Mezimim who tried to kill Tereifos. Even so, the Torah is stringent to kill them through such testimony.

Ohr Some'ach (ibid.): If a Tereifah and a healthy person testified in a capital case, we accept their testimony. If they are Huzmu, they are killed. Testimony of the Mezimim is Efshar Lehazimah, for they are liable for convicting the healthy witness. However, if witnesses testify that a Tereifah (Ploni) had Bi'ah with Almoni's wife, this is Iy Efshar Lehazimah, for they would be killed for trying to kill her. The testimonies about Ploni and her are separate. They just happened to testify about both of them. This is unlike Hazamah, for witnesses are punished only if both were Huzmu. When both are Huzmu, it is one testimony. However, Tosfos (Yevamos 25a DH Mes) disagrees. The Ri explains that the Gemara asked that the murder victim should save the murderer because he became a Tereifah, who is a Pasul witness. I showed that a Tereifah is a Kosher witness, and joins with a healthy witness! There are many other Perushim of that Sugya. However, "Shnei ha'Anashim" equates the witnesses. Since two Tereifos cannot testify, perhaps a Tereifah cannot join with a healthy witness.

Avnei Nezer (CM 80 DH Nistapakti): When one of the witnesses is Tereifah, the testimony is Efshar Lehazimah. However, perhaps even so the testimony is invalid, for more than half is due to the healthy witness (if not for him, also the Tereifah is invalid). Chelkas Mechokek (35:3) says similarly (when one witness would be partial if not for the other witness).

Ohr Some'ach (Hilchos Edus 20:8): If witnesses gave testimony that was Efshar Lehazimah, and after it was accepted it became Iy Efshar Lehazimah, e.g. the Nidon became Tereifah before the final verdict, what is the law? Since they are not liable until the final verdict, this is not "Nefesh b'Nefesh", so they could not be killed. However, perhaps we kill the Nidon, for when they testified it was Efshar Lehazimah. Rashi (75b DH Heicha) says that Iy Efshar Lehazimah disqualifies only when they do not know the day, for it seems that they fear Hazamah. We do not disqualify testimony because it is surely true. If so, here the testimony is true, for it was Efshar Lehazimah when they testified. If we would not say so, anyone convicted to pay a fine would admit to the witnesses after Drishah v'Chakirah, so at the time of the final verdict it is Iy Efshar Lehazimah! However, perhaps it is Efshar Lehazimah, for if they are Huzmu, he can say that he admitted only to Mevatel their testimony and make it Iy Efshar Lehazimah. This is why the Gemara discusses one who said 'I stole, but not in front of you, rather, in front of Ploni and Almoni.'

Poskim

Rema (33:16): Relatives of the murder victim can testify about the murderer.

Mordechai (Sanhedrin 695): Relatives of the murder victim can testify about the murderer. Even the victim can testify, as long as he is not Tereifah. A man can testify 'Do'eg had Bi'ah with me' to kill Do'eg.

Beis Yosef (CM Sof Siman 33 DH Kosvu ha'Mordechai): This implies that a Tereifah cannot testify.

Darchei Moshe (9): This is only for capital cases. The Nimukei Yosef wrote that we do not require Efshar Lehazimah for monetary cases.

Shulchan Aruch (17): Witnesses related to the judges are Pesulim.

SMA (26): The Mordechai and Nimukei Yosef hold that only capital cases require Efshar Lehazimah. They allow witnesses who are related to the judges or are Tereifos, for monetary cases. The Rema omitted their opinion because many disagree.

Rebuttal (Shach 16): This is astounding. Testimony of a Tereifah in monetary cases can be Huzam! The Beis Yosef connotes that a Tereifah is intrinsically Pasul for testimony. He and the Darchei Moshe overlooked Sanhedrin 78a. Rava and Rav Ashi agree that intrinsically, a Tereifah can testify even about capital cases. The only concern is Efshar Lehazimah.