Bava Batra 32: A Counterfeit IOU
Our Gemara cites a mishnah from Ketubot regarding not determining status, specifically of a kohen, when there is testimony from only one witness - with caveats and those who say we do trust the one witness, depending. Refining the dispute - can witness testimony be combined (that is, when the witnesses are not testifying together). Plus, a dispute over an IOU, which turns out to be forged - though the debt is real.