Keritot 12 - Witnesses Tell Him That He Inadvertently Ate Forbidden Fat
If one witness tells somebody, "You ate forbidden fat," and the man remains silent, he has to bring a sin-offering. However, if he denies it, he is not liable. If two witnesses tell a man that he ate forbidden fat, and he denies it, then Rabbi Meir holds him liable, but the Sages exempt him.
(There are two sorts of animal fat, permitted, called "shuman," and forbidden, called "chelev". If one eats forbidden fat by mistake, he needs to bring a sin-offering, and if he eats it knowingly, he is liable to be cut off from his spiritual source.)
What is the source for these laws and what is the logic of the argument? The Torah said, " If his sin becomes known to him ." This means that he himself became aware of his act. Alternatively, others told him, but he was silent, thus confirming that he accepted the fact and became aware of it. Thus, if he denies one witness, he is believed.
When two witnesses tell him that he ate the wrong fat, we have an established fact. Rabbi Meir then reasons that if witnesses can testify about a capital crime and have a man executed, then certainly they can obligate him to bring a sacrifice. The Sages, however, say that he has a way out: he can claim that he ate the fat on purpose, and be free from a sacrifice obligation. This claim, of course, does not work to escape capital punishment.
Art: Hermann Kern - The Peasant's Meal