Ketubot 19A

חזקה אין העדים חותמין על השטר אלא אם כן נעשה בגדול. אלא אנוסין, מאי טעמא?

There is a presumption that witnesses sign on the document only if the transaction was made when both parties to the transaction are adults. A corollary of that presumption is that each party would sign only adult witnesses to the document. However, if their testimony was that they were compelled to sign the document, what is the reason that R' Meir rules that their testimony is not accepted?

אמר רב חסדא, קסבר רבי מאיר: עדים שאמרו להם ״חתמו שקר ואל תהרגו״⁠ — יהרגו ואל יחתמו שקר.

Rav Ḥisda said that R' Meir maintains: Witnesses that others said to them: Sign a document containing a falsehood and you will not be killed, should allow themselves to be killed and they should not sign a document containing a falsehood. Therefore, even when they testify that they were compelled to sign the document due to a threat to their lives, they are incriminating themselves.

אמר ליה רבא: השתא אילו אתו לקמן לאמלוכי, אמרינן להו: זילו חתומו ולא תתקטלון, דאמר מר: אין לך דבר שעומד בפני פיקוח נפש אלא עבודה זרה, וגלוי עריות, ושפיכות דמים בלבד. השתא דחתמו, אמרינן להו: אמאי חתמיתו?

Rava said to him: Now, if the witnesses came before us to consult with the rabbis, we say to them: Go sign the document and you should not be killed, as the Master said: You have no matter that stands before saving a life, other than idol worship, forbidden sex, and murder. Now that they signed, do we say to them: Why did you sign? Only in those 3 cases, when faced with a choice between violating the prohibition and being killed, must one be killed rather than violate the prohibition. Signing a false document does not fall into that category. Why then, according to R' Meir, is their testimony that they were compelled to sign the document not accepted?