The Mishna says [Ketuboth 18b] if two witnesses say the signature on a doc is theirs, but that they were invalid witnesses at the time, they are believed [unless their signature can be validated from another source.] But Rav Nachman [Ketuboth 19b] said if they say it is their signature, but that the doc had an oral agreement that would invalidate it, they are not believed. The problem here is that in the first case they are believed because the same statement that validates it (but admitting it is their signature) is the same statement that invalidates it. Their claim is the signatures are invalid. But that same reasoning ought to apply to the second case. But we do not believe them in the second case. What is the difference? Tosphot asks this on page 19. Reb Aaron Kotler explains Tosphot as intending to say this: Where we do believe them, it is because they say the doc was not valid by law. In the second case, we do not believe them because they admit the doc was valid, but that there was an outside factor that invalidates it. The problem I see in this is that it seems different from what Tosphot says. On page 18, Tosphot says we believe them because we need them to establish the validity of the doc, and that is lacking. Tosphot did not say the reason we believe them is that they completely invalidate the doc. Lack of validation is not the same thing as invalidating it.=====================================The Mishna says [כתובות 18b] if two witnesses say the signature on a שטר is theirs, but that they were invalid witnesses at the time, they are believed [unless their signature can be validated from another source.] But רב נחמן [כתובות 19b] said if they say it is their signature, but that the שטר had an oral agreement that would invalidate it, they are not believed. אם אומרים שהיה שטר אמנה אינם נאמנים The problem here is that in the first case they are believed because the same statement that validates it (but admitting it is their signature) is the same statement that invalidates it. Their claim is the signatures are invalid. But that same reasoning ought to apply to the second case. But we do not believe them in the second case. What is the difference? תוספות asks this on page 19. רב אהרון קוטלר explains תוספות as intending to say this: Where we do believe them, it is because they say the שטר was not valid by law. In the second case, we do not believe them because they admit the שטר was valid, but that there was an outside factor that invalidates it. The problem I see in this is that it seems different from what תוספות says. On page 18, תוספות says we believe them because we need them to establish the validity of the שטר, and that is lacking. תוספות did not say the reason we believe them is that they completely invalidate the שטר. Lack of validation is not the same thing as invalidating it.