Responsa for Yevamot 201:2
והתניא רבי יהודה אומר פטור בבת אחת תרי תנאי אליבא דרבי יהודה
R. Judah. however, said: If<span class="x" onmousemove="('comment',' He struck or cursed. ');"><sup>4</sup></span> simultaneously, he is guilty;<span class="x" onmousemove="('comment',' The specific caution that must precede any forbidden act that is punishable by a court is here effected when the witnesses cautioned the offender by one statement against the striking or the cursing of the two, e.g., 'do not strike them'. ');"><sup>5</sup></span> if<span class="x" onmousemove="('comment',' He struck or cursed. ');"><sup>4</sup></span> successively he is exonerated.<span class="x" onmousemove="('comment',' Though he may have been duly cautioned in each particular case, no penalty can be imposed upon him by any court, since each caution was of a doubtful character, it being unknown in each case whether the particular man he was about to strike or curse was his father or not. A caution of a doubtful character is, in the opinion of R. Judah, of no validity. while in the opinion of the first Tanna it is valid. ');"><sup>6</sup></span>
Teshuvot Maharam
A. Bediabad, in this case, means after the rite was performed. Nevertheless, the aforementioned performance of the rite is invalid, for a different reason: the shoe used was sewed with flaxen thread and, therefore, was never fit for this purpose. Since, however, the levir performed an invalid rite of halitzah, neither he nor his brothers are now permitted to marry the widow. Halitzah by the same levir being the only manner of releasing her, he may be forced, by persuasion or by flagellation, to repeat the rite with the proper shoe. For now the Mitzvah revolved on him alone, and we are permitted to scourge a Jew until he perform the required Mitzvot.
This Responsum is addressed to Rabbi Menahem of Würzburg.
SOURCES: Am I, 93, 94.