In the past (here and here) we have discussed the machloket between the Shach and Bach (both in the Kelalei Psak of the Shach- Yoreh Deah 242) whether the rule acharei rabim lehatot applies only to cases of Beit Din or to all cases of pesak. The Shach argues that anything that has been ruled on by a majority of poskim throughout the generations becomes binding pesak and cannot even be invoked in a shaat hadechak. The Bach disagrees, arguing that only in Sanhedrin, where there was a formal vote, can we invoke rules of rov.
Both R. Yaakov Emden and R. Yonatan Eibshitz notes that practically the Shach is impossible to implement, as we have no way of knowing how many sefarim have been lost, how poskim have rules orally, etc, making it impossible to reach a rov. Continue reading Does Majority Rule Apply Outside of Halachic Issues?