Nedarim, Chapter 11, Mishnah 5. This Mishnah discusses a man who annulled his wife's vow or his daughter's vow based on a mistaken understanding of what the vow actually was. In such a case the annulment is not valid, and he may still annul the vow even at a later time, because he is not considered to have heard the vow at all. The rule is that he has only one day from the moment he hears the vow, but here he never really heard the vow as it was.
The text of the Mishnah:
"If his wife made a vow and he thought it was his daughter who made the vow, if his daughter made a vow and he thought it was his wife who made the vow, if she took a vow of nezirus and he thought she made a vow of korban, if she made a vow of korban and he thought she took a vow of nezirus, if she vowed against figs and he thought she vowed against grapes, if she vowed against grapes and he thought she vowed against figs - he must go back and annul the vow again".
The Mishnah lists three types of mistake:
A mistake about who made the vow: his wife made the vow and he thought it was his daughter who made it, or his daughter made the vow and he thought it was his wife who made it.
A mistake about the type of vow: she took a vow of nezirus, forbidding herself wine and haircutting and the like, and he thought she had made a vow of korban, meaning that she forbade something to herself like a korban or undertook to bring a korban; or the reverse, she made a vow of korban and he thought she had taken a vow of nezirus. On the basis of his mistaken understanding he annulled the vow.
A mistake about the subject of the vow: she vowed not to benefit from figs and he thought she vowed against grapes, or she vowed against grapes and he thought she vowed against figs.
In each of these cases "Harei zeh yachzor v'yafer" - he must go back and annul the vow a second time, and only then does the annulment take effect. Even if he already annulled the vow on that same day, since he did not understand the vow correctly his annulment is no annulment, and he is permitted to annul it again on that very same day, from the moment he grasped its true content.
In summary: in this Mishnah we learned that an annulment made in error, whether about who made the vow, the type of vow, or the subject of the vow, is not an annulment, since the husband or the father never heard the vow as it actually was. He therefore goes back and annuls it once the content of the vow becomes clear to him, and the count of the day begins from the moment the vow became properly known to him.