Ketubot, Chapter Two, Mishnah Three
This mishnah contains another case illustrating the principle of the mouth that forbade is the mouth that permits.
1) If witnesses said, This is our handwriting, but we were forced, [or] we were minors, [or] we were disqualified witnesses they are believed.
2) But if there are witnesses that it is their handwriting, or their handwriting comes out from another place, they are not believed.
Section one: In this scenario, a person comes to court with a document signed by witnesses. When his opponent claims that the document is a forgery, the witnesses are summoned to the court to testify to their signatures. The witnesses state that the signatures are indeed their signatures, but that nevertheless the document should not be upheld. This is for one of three reasons: they were forced to sign, they were minors when they signed, or they were disqualified witnesses (see Sanhedrin 3:4). In this case they are believed, and the document is invalid. This is because of the principle of the mouth that forbade is the mouth that permits. Without the witnesses admission that they signed the document, the document would have been invalid. When they admit that they signed, they are in fact the mouth that forbade. When they say they were forced, or that they were minors or otherwise disqualified, they are the mouth that permits, and they are believed. To state this another way, if they had wanted to lie they could have said that this was not their handwriting.
Section two: If their signature can be validated in another way, for instance by other witnesses testifying that they recognize the signatures, or by another document that contains their signatures, then the witnesses are not believed when they say that they were forced, or they were minors or otherwise disqualified. This is not a situation where the mouth that forbade is the mouth that permitted. Since they are not believed to say that they were invalid, their signatures are validated and the document is upheld.