Responsa על עבודה זרה 147:3

Rabbi Samuel of Eisenach, a relative of R. Meir, permitted the eating of ritually slaughtered chickens with which a trefah chicken was mixed up. He based his decision on an opinion he said he had received from R. Jacob, that the law stated in Mishna Ab. Z. 5, 9, and interpreted in the Talmud Ab. Z. 74a, is the accepted law governing mixtures. This talmudic interpretation implies, that only objects possessing both characteristics: a) items usually sold by the piece, and b) forbidden objects, the profitable use of which is prohibited (issurei hanaah), are never neutralized in a mixture. However, objects lacking one of these characteristics do become neutralized in the proper mixture.
R. Meir took issue with R. Samuel, pointing out, that the accepted authorities never mention the foregoing view as the final law. On the contrary, these authorities decide that items usually sold by the piece, even though they be not forbidden objects the profitable use of which is prohibited, are never neutralized. Moreover, R. Samuel contradicted himself and ruled that a "piece fit to be offered a guest", even though it be not issurei hanaah, is not neutralized; but a whole chicken is also considered "a piece fit to be offered a guest". R. Meir, therefore, pleads with R. Samuel to acknowledge his mistake, and change his decision, lest it become a misleading precedent which may be followed by future generations.
SOURCES: Cr. 14; Am I, 75.