Nedarim 66 - I will not marry this girl!
Initially the rule was that if a part of a vow is annulled, one still has to fulfill the rest of it. Let's say someone vowed against drinking wine, or eating meat. They tell him, did you realize that on Shabbat and Yom Tov one is not supposed to deprive himself? He says, "Had I known that, I would never have vowed" - and he can drink on Shabbat.
However, Rabbi Akiva extended this: once is permitted to drink on Shabbat, he is also permitted to drink on other days, the vow is void. How did Rabbi Akiva derive that? - From the phrase, "The full vow that you pronounce you should fulfill." The other side of this is: once you don't have the "full vow" obligatory on you, you don't even have a part.
If one says, "I am not marrying this girl because she is ugly" and she turns out to be beautiful, his vow is annulled, because it was a mistake. We assume that he just did not see her beauty at first.
Rabbi Ishmael takes this further: even if she was ugly and was made beautiful - so that this is something unforeseen, not something that he did not realized - his vow can still be annulled. Once a man vowed that he will not marry his niece because she was ugly (she had a bad-looking false tooth). Rabbi Ishmael paid the dentist to fix it, provided beautiful clothes to her, and then asked the man, "Is it this one that you vowed against?" The man said, "Had I known that she will be so beautiful, I would never have taken the vow!" Rabbi Ishmael cried and said, "All daughters of Israel are beautiful, it is only poverty that makes them sometimes homely."
Art: Company drinking tea and wine in an inn by Heroman Van Der Mijn