Gemara

Question: "She (Tamar) sent to her father-in-law 'I am pregnant from the owner of these.'" Why didn't she say explicitly that it was from Yehudah?

Answer (Rav Zutra bar Tuvya): We learn from her that it is better to cast oneself into a furnace than to be Malbin Pnei Chavero (whiten someone's face) in public.

Bava Metzi'a 58b (A reciter of Beraisos): Malbin Pnei Chavero is like bloodshed.

Support (Rav Nachman bar Yitzchak): Yes! After (he blushes), the redness fades and the face turns white again.

(R. Chanina): Everyone who goes to Gehinom leaves, except for three: Malbin Pnei Chavero...

Mishnah (Avos 3:11): Ha'Malbin Pnei Chavero has no share in the world to come.'

Erchin 16b (Beraisa): If one sees someone do something improper, he must rebuke him, for it says "Hoche'ah Tochi'ach." "Tochi'ach" teaches that if he did not accept the rebuke, he must rebuke him again.

Suggestion: Perhaps this is even if the sinner will blush from embarrassment!

Rejection: "V'Lo Sisa Alav Chet."

Yoma 86b: We publicize flatterers due to Chilul Hash-m. It says "when a Tzadik leaves his righteousness, I will put a stumbling block in front of him."

Rishonim

The Rif and Rosh (Bava Metzi'a 33a and 4:22) bring the Gemara in Sotah and Bava Metzi'a.

Rashi (Yoma 86b DH she'Ne'emar): When a Tzadik sours, Hash-m presents a sin for him to stumble in publicly, so all will know about his deeds.

Rambam (Hilchos De'os 6:8): One who shames another is not lashed, but it is a great sin. Chachamim taught that ha'Malbin Pnei Chavero has no share in the world to come. Therefore, one must be careful not to shame a minor or adult in public. One may not say in front of him something he is ashamed about. This refers to interpersonal matters. In matters of Shamayim, if one does not repent in private, we shame him publicly, publicize his sin, revile him in his presence and disgrace him and curse him until he repents, like all Nevi'im did to Yisrael.

Question (Lechem Mishneh): This connotes that we may not shame one for interpersonal matters. Perhaps he means that we may not shame him in public. It does not seem this way. Also, what is the source to distinguish?

Answer (Minchas Yitzchak, Hagahos on Sefer ha'Chinuch 240:3): He learns from Yoma 86b, which says that we publicize flatterers due to Chilul Hash-m.

Minchas Chinuch (240, DH v'Nir'eh): It seems that one may not shame Ploni for something that Ploni did to him. It is better to forgive him. He may shame him for something that Ploni did to others. The Nevi'im rebuked for matters Bein Adam l'Chavero!

Kometz Minchah (240): I am unsure whether one may shame someone for a sin mid'Rabanan Bein Adam la'Makom. Chachamim did not enforce their laws to override Torah law through actions, e.g. shaming someone.

Sefer ha'Yir'ah (DH Im Tochi'ach): If one rebuked someone in private several times and he did not accept it, one may shame him and be Malbin Panav until he accepts.

Sha'arei Teshuvah (3:219): If one cast off the yoke of Shamayim and is not careful about one Aveirah that everyone knows is forbidden, one may shame him for his deeds, publicize his abominations, and pour disgrace on him. We publicize flatterers due to Chilul Hash-m.

Chafetz Chaim (Hilchos Lashon ha'Ra 4, Be'er Mayim Chayim 30): If Ploni repeatedly sins, Levi may reveal his detriment, so that people will not go in his ways. Ploni is not 'your countryman'; he is a flatterer. We are not concerned lest he already repented. People will not suspect Levi, for they will see that Ploni is a Rasha. Levi must not intend to enjoy speaking badly about him, or speak out of hatred, only for the sake of the truth.

Rambam (Hilchos Teshuvah 3:14): There are Aveiros about which Chachamim said that one who does them regularly has no share in the world to come: ha'Malbin Pnei Chavero in public... This is if he died without Teshuvah. If he repented, he gets the world to come. Nothing stands in front of Teshuvah.

Perush ha'Mishnayos (Avos 3:11): Malbin Pnei Chavero is shaming someone.

R. Yonah (ibid., and Sha'arei Teshuvah 3:139): This is an offshoot of murder, for which one must be killed and not transgress. The redness leaves his face, like murder. Also, the pain of shame is worse than death. Chachamim did not say that it is better to cast oneself into a furnace than to transgress other Aveiros!

Bartenura (ibid.): When one is shamed, initially his Ru'ach moves outwardly, like one who is angered and his face gets red. When he finds no assertion to avert his shame, he worries internally and his Ru'ach retreats inward due to pain, and his face whitens.

Tiferes Yisrael (ibid., 72): Such a person denies that man was made in Tzelem Elokim and that his Neshamah is eternal. Therefore, he is not concerned to honor people. Yayin Levanon says that one who shames one whom he hates at the time of anger is like anyone else who is enticed by his Yetzer to sin. Rather, ha'Malbin Pnei Chavero, i.e. he disgraces one who did not do evil to him, is an Apikorus for denying the Tzelem Elokim.

Poskim

Shulchan Aruch (420:39): Even though one does not pay for verbal embarrassment, it is a great sin. Only an insane, haughty Rasha would do so. Ha'Malbin Pnei Chavero has no share in the world to come.

Magen Avraham (156:2): If Ploni sinned against Shimon, Shimon should not hate him and be quiet. Rather, he should say 'why did you do so to me?' He may not speak harshly to the point of shaming him. Ha'Malbin Pnei Chavero has no share in the world to come. Rather, he rebukes him gently in private. If he does not change after two or three times, he publicly shames him.

Question (Eshel Avraham): This is from the Rambam (Hilchos De'os 6:6-8). The Rambam says so only about sins Bein Adam la'Makom, and the Magen Avraham says so Stam! The Rambam (Reish Hilchos De'os) says that ha'Malbin Pnei Chavero, even in private, transgresses "Lo Sisa Alav Chet." One must be extra careful in public, for then has no share in the world to come. Rashi (Vayikra 19:17) says that "v'Lo Sisa Alav Chet" forbids ha'Malbin Pnei Chavero in public. I.e., do not rebuke someone in public, for then even if you speak softly, you will shame him.

Shulchan Aruch ha'Rav (156:8): We do not shame one for a sin in private. If one sins openly, we rebuke him immediately to avoid Chilul Hash-m.