Bava Batra 49 - Witnesses Can't Change their Minds
Tabbi distressed Pappi until he relented and sold him what he demanded. Rabbah bar bar Chanah signed two documents: notification that the sale was executed under duress, as well as the document of sale
Said Rav Huna: both documents have legal force. But how can it be? Rather, what Rav Huna meant is that the forced sale would be valid, if not canceled by the prior notification - since it was Rav Huna who introduced the forced sale rule .
But we have a rule that a witness, having testified, can't offer a new testimony contradicting his first one . How could Rabbah sign on both documents? - Exactly! Since he signed on the notification of duress first, his signature on the document of sale becomes invalid.