Kesubos 21a. When a witness signed on a loan document needs to verify his signature, he can write it on a potsherd and give it to the Beis Din and they will compare it to the signature on the document. Only on a potsherd, but not on a parchment, lest a criminal find it and write a loan document above the signature, as it says in the Mishnah (Bava Basra 175b): If the lender produced a document signed by the borrower that he owes him, he may collect from the borrower’s property, provided that the property has not been sold to anyone else.
כתובות כא ע”א: ודוקא אחספא, אבל אמגלתא לא, דלמא משכח לה איניש דלא מעלי וכתב עילויה מאי דבעי, ותנן: הוציא עליו כתב ידו שהוא חייב לו, גובה מנכסים בני חורין.
In the autumn of 2001 after suffering a stroke, Rav Chaim Kanievsky was recuperating in the Mayenei Hayeshua Hospital in Bnei Brak. His therapist asked Rav Chaim to write something on a sheet of paper to accustom him to once again using his hands and fingers. Slowly and painstakingly, he wrote these words, “Yekum purkan min shemaya.” The therapist was taken aback. “I have been doing this work for years,” he said to the great sage. “Every single person – without fail – whom I ask to write something, always signs his name. Why did the Rov not do that?” Rav Chaim answered, “Chazal teach that a person should not write his name on a blank sheet of paper, lest a dishonest person find and write above the signature that the undersigned owes him money, and then he would be liable.”
According to another version, this story happened after Rav Chaim was awakened from anesthesia by his doctor. The doctor gave him a blank piece of paper and told him to sign his name, in order to see if he was fully alert. Rav Chaim wrote the letters ב”ב קעה ע”ב – the place where the Mishnah says that one’s signature may be used as a proof to collect money from him.
Source: FJJ 3/24/2022, p. 14 and p. 36.