beitza or bay'ah

Many have the custom to call this mesechta by the name bay'ah rather than beitza. This is based on the Magen Avraham 156 in the name of maharshal that we should not say a davar megunah. The meforshim on the mishnayos, however, point to a different source. The gemara in Sanhedrin 5b says that there was a town that mistakenly assumed mei betzaim (swamp waters) are not machshir l'kabel tumah. A talmid chacham had told them that "mei beitzim" are not machshir l'kabel tumah. Some of the meforshim point out that clearly the custom was to call it beitza or else there would have been no room for error. (bay'ah and betzaim do not sound alike and would not have been confused). Others (Tiferes Yisrael) point out that maybe because of the confusion that occured they decided not to call it beitza anymore and that is where the custom to call it bay'ah really originated. See Mishnah Rishonah there.