My chavrusa and I were going through Sanhedrin (40b) this evening, and the topic of how to get accurate statements from witnesses. Rashi mentioned the concept of moving the witnesses around in order to get them "confused" so that they'll state the truth:
כר״ש בן אלעזר. להטריחם דאמר (לעיל לף לב:) מסיעין היו את העדים ממקום למקום כדי שתטרף דעתן עליהן ויחזרו בהן׃
My chavrusa and I started talking about this practice, and we were wondering whether there is any recognition in the gemara that coerced testimony may not be valid. I'm familiar with the concept of "כפין אתו עד שאמר רצה אני", but I don't think this is similar... that concept is uniquely suited to situations where there is a mitzvah being done. Here, he is simply giving testimony about a case. Is there a parallel in halacha?