Did Yishai sin? The gemarah (shabbat 55b) states pretty clearly that he did not; however, the midrash (Yalkut HaMakiri, cited here) seems to indicate that he separated from his wife for an extended period of time, which is forbidden. How do we resolve these two statements?
According to the version cited in Gershon Gold's answer, the reason why he separated was because they hadn't revealed the law of מואבי ולא מואבית, and his ancestress was from Mo'av (meaning he was forbidden from marrying a Jew). I also saw this version quoted in Sefer HaToda'ah in the month of Sivan, but I don't have the book in front of me to check again.