Last night I was given a possible justification for the infamous Kaminetsky-Greenblatt Heter. It was claimed that the Yerushalmi (Yevamos 15:4) states that once a beis din has given a psak that a woman could remarry - the heter remained valid even though the basis for the psak was lies and a misunderstanding of elementary halacha.
Then the husband comes to court - and the court denies that he is the husband and beats a talmid chachom who suggests otherwise. Thus it seems that a mistaken psak can validate a second marriage as we have in the case of Tamar Epstein.
Yehuda added this from the Bavli
(Yevamos 88a):MUST . . . LEAVE THE ONE AS WELL AS THE OTHER etc. Rab stated: This was taught only in respect [of a woman] who married on the evidence of a single witness, but if she married on the strength of the evidence of two witnesses, she need not leave.44 In the West45 they laughed at him. Her husband [they remarked] comes, and there he stands, and you say: She need not leave! This46 [it may be replied] was required only in the case when the man47 was not known.48 If he49 is unknown, why is she to leave [her second husband] even where she only married on the evidence of a single witness? This is required only in the case where two witnesses came and stated, We were with him49 from the moment he left until now, but you it is who are unable to recognize him;50
Regarding Rav's statement in the Yerushalmi that even if the husband comes back we don't remove a Heter given based on two witnesses: the Bavli Yevomos 88a brings it and says it only applies when we don't recognize the husband, and in fact in Eretz Yisrael they laughed at the thought that we would allow her to stay married if the husband is here. Tosafos explains that the husband returning is far better then two witnesses. Even the Yerushalmi itself in Yevomos 10'5' according to Korbon Ha'eidah's text says that we do not pasken like Rav because he is contradicted by a Mishna. The Ramban on the Parsha of Eidim Zomimin in Shoftim says that the dead person returning cannot make the pair of witnesses into zommimin because the posuk only refers to cases that require some investigation, but if the man returns then it is obvious that the witnesses lied.