Reference for Ketubot 199:12

לא דכ"ע טעה בדבר משנה חוזר ומר סבר הכי הוה מעשה ומר סבר הכי הוה מעשה

And one master holds it is not reversed. No; all agree that if one errs in a decision based on a mishnah, the ruling is reversed, but one Master holds that the incident occurred in one way while the other holds that it occurred in the other way.

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