While the first two chapters of Massekhet Karetot focused on defining the transgressions whose violation would obligate the individual to bring a korban ḥatat – a sin offering – the third perek that begins on today’s daf discusses the conditions necessary for that obligation to take effect. For example, the Torah (Vayikra 4:28) requires that the sinner be aware of his transgression, but in many cases he will be made aware of this only when others inform him of what he did. Is this true testimony that requires two witnesses, or will just one suffice? What if the witness is someone whose testimony ordinarily is not acceptable in court?
The Mishna on today’s daf teaches that if witnesses testify that a person ate forbidden fats, he is obligated to bring a sin offering. If a single witness testified that he did so, he would not be obligated in a sin offering. If one witness said that he ate forbidden fats and another witness contradicted that testimony, he will bring an asham taluy – a provisional guilt offering (that is brought when it is not clear whether or not the circumstances require a sin offering).
What if the person denies the testimony of the witnesses and insists that he did not sin? Rabbi Meir requires him to bring a sin offering, arguing that their testimony would have been relied upon by the court to mete out a death sentence (had they testified that he had sinned intentionally), so they should certainly be believed to obligate him to bring a sacrifice.
From other statements made by Rabbi Meir, it appears that he believes that the court forces him to bring the sacrifice. Tosafot point out that although according to Jewish law we never force someone to bring a sin offering, that is only because we do not force someone who is uninterested in atonement to bring a sacrifice. This case is different, since this individual does want atonement; he is just under the impression that he needs no atonement, since he has not sinned. The court, which accepts the testimony of the witnesses, believes that he has sinned and requires the sacrifice.