Wed Nedarim 66

The Tana Kama says that if he makes a Neder from this ugly woman and then finds out she was really beautiful, he made the Neder mistakenly so the Neder was never valid. However, if at that time she was ugly, but she developed later to become beautiful, she remains Assur to him.

Ran (before) asks, that we already said that if he makes a Neder on a woman because of his dastardly father, he's Muttur to her if her father dies or does T'shuva. We say that it's as if he made a condition, and as long as the reason is no longer valid, so the Neder is no longer in effect. Why dont' we say the same here?

The Rashba answers: over there, it's apt for the father to either die or do T'shuva, therefore we say he had in mind that the Neder should only last while the father is still a bum. However, it's not to often that the ugly turn beautiful, so we say he meant to Assur her completely.

However, R' Shmuel answers: that if he would say he makes a Neder from her because she's ugly, then when she becomes beautiful she's Muter, since it seems that it's a condition, since he gives it specifically as the reason she's Assur. However, our case is that he makes a neder from this ugly woman. So we don't see he's giving a reason, but rather just identifying the women. Therefore we assume he Assurs her forever.