Rashbam on Exodus 23:7:1

מדבר שקר תרחק - ואם נראה בעיניך דין מרומה ועדים רמאים ואין אתה יכול להכחישן, התרחק מאותו הדין ואל תדין בו כלל. ואעפ"כ אם הוא נקי וצדיק בדין, שכבר נגמר הדין לזכות אל תהרגנו עוד, מאחר שכבר קבלת העדים וגמרת הדין ולא נתת לבך על זה, עד לסוף שאם אתה הצדקתו אני לא אצדיקנו וחייב מיתה, כדכתיב: והאלהים אנה לידו. מדבר שקר תרחק, [these words are addressed to the judges Ed.] if you feel that that a perversion of justice is about to take place, aided and abetted by witnesses who perjure themselves, disqualify yourself from participating or presiding in such proceedings unless you know that you can convincingly prove the perversion of justice about to take place.
ונקי וצדיק, if the guilty party has been exonerated at his trial you cannot subsequently “kill him,” i. e. reopen the proceedings seeing that you had accepted the testimony of the witnesses at the trial and brought the trial to a close. The reason is that if you mistakenly exonerated a guilty party, I. G’d, am the last recourse and will see to it that the guilty party will not remain perceived as righteous. We have had Exodus 21,13 והאלוהים אנה לידו as an example of G’d taking corrective action, causing someone who had been wrongly exonerated to be killed by his fellow man (though inadvertently).