Makos 3b-4a: shloshes lugin mayim... is the girsa "shloshes lugin mayim chaser kortav" or does thee mishna not include the words chaser kortav?
R. Tzvi Hirsch Grodzinsky in Mikve Yisrael, Anaf Chamishi asks: We hold like Rava who is the later shita, and he says that the girsa is chaser kortav, and we pasken like rabanan who are the majority opinion, then we should pasken 3 complete lugin that a kortav of wine fell into them and they look like wine, should invalidate the mikva like the rabanan. Why don't we pasken that way? I think this is also the question of Tosfos Rabi Akiva Eiger, Mikvaos Perek 7, os 12.
Mikve Yisrael answers that we pasken like Rav in Issurim, so whichever way he paskens - whether like Rabanan or Rabi Yochanan ben Nuri, we'd pasken like Rav. Also, Chullin 26a implies Rava paskens like Rabi Yochanan ben Nuri by wine - so we pasken like Rabi Yochanan ben Nuri by wine, and because it is not clear that Rava paskens like Rabi Yochanan ben Nuri by milk, we don't pasken like him, and instead pasken like Chachamim in that case.