Reference for Bava Batra 327:2

וליחוש דלמא שדי דיותא אמקום עדים מעיקרא ומחיק ליה דכי הדר מחיק ליה להאי הוה ליה אידי ואידי נמחק שתי פעמים

But [is there no cause] to apprehend that ink might first be poured on the place of the witnesses,<span class="x" onmousemove="('comment',' I.e., on the lower section (corresponding to the place of the witnesses) of a paper which has been once erased from top to bottom. ');">⁴</span> and this<span class="x" onmousemove="('comment',' The ink poured. ');">⁵</span> would be erased,<span class="x" onmousemove="('comment',' And thus the witnesses, not suspecting that the section where they append their signatures had been erased twice, whereas the upper section only once, would be signing on a double erasure. ');">⁶</span>

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