The Mishnah (Makkos 5b) teaches that when one member of a group of witnesses (kat eidim) is disqualified from testifying – for example, if he is related to another witness or the subject of the testimony – the entire group of witnesses is disqualified:
ומה שנים נמצא אחד מהן קרוב או פסול – עדותן בטלה ,אף שלשה נמצא אחד מהן קרוב או פסול – עדותן בטלה .מנין אפילו מאה ?תלמוד לומר ״עדים ״ .
And just as with regard to two witnesses, if one of them is found to be a relative or is otherwise disqualified, their entire testimony is voided, as it is no longer the testimony of two witnesses, so too, with regard to three witnesses who came to testify as one set – if one of them is found to be a relative or is otherwise disqualified, their entire testimony is voided, even though two valid witnesses remain. From where is it derived that the same halachah applies even in the case of one hundred witnesses? It is derived from a pasuk, as the pasuk states: “witnesses”.
The Gemara (Makkos 6a) qualifies this statement:
אמר ליה: ״יקום דבר״ – במקימי דבר הכתוב מדבר.
Story With the Birchas Shmuel
There is a story with R’ Boruch Ber Lebovitz, the Ba’al Birchas Shmuel, that he was once asked to be a witness for the giving over of a get, and he suddenly remembered that he once borrowed a sefer and he hadn’t yet returned it. R’ Boruch Ber was worried that perhaps he violated the sin of theft, and he was unsettled until he was mekabel on himself to return the sefer as soon as he could. He then had hirhuray teshuvah, and finally agreed to be the eid.
