Rabbi Hiya bar Aba was in a place called Gavla - which place that is specifically is unclear, though suggestions are made - where he saw Jewish young women who were impregnated by men who were in the process of converting to Judaism. Also re their wine. Likewise, he saw Jews eating from bishul akum. He said nothing, but consulted with Rabbi Yochanan later, and he said that the babies must be proclaimed mamzerim, the wine - yein nesach, and so on. Because this population was just not that knowledgeable. Also, how does someone whose wine has become prohibited via a non-Jew's libation, how can that Jew recoup the loss? Isn't that wine prohibited from benefit? That turns out to be a matter of dispute.