Daf TB Menakhot 2 cites Rabbi Shimon in two different baraitot which contradict each other.

Baraita #1 “Rabbi Shimon says: All the meal offerings from which a handful was removed not for their sake are fit for sacrifice and they even satisfy the obligation of the owner. The baraita continues: Conversely, consecrated animals that were sacrificed not for their sake do not fulfill the obligation of the owner, as in this regard meal offerings are not similar to slaughtered offerings. The difference is that when one removes a handful from a pan meal offering for the sake of a deep-pan meal offering, its mode of preparation proves that it is in fact for the sake of a pan meal offering, as the two offerings differ in appearance. Similarly, with regard to a dry meal offering, e.g., the meal offering of a sinner, which contains no oil, whose handful is removed for the sake of a meal offering that is mixed with oil, its mode of preparation proves that it is for the sake of a dry meal offering, and one’s improper intent is therefore disregarded. But with regard to slaughtered offerings it is not so, as there is one manner of slaughter for all offerings, and one manner of sprinkling the blood for all offerings, and one manner of collection of the blood for all offerings. Since the differentiation between slaughtered offerings is established only through intention, one who sacrifices an animal offering not for its own sake does not fulfill the obligation of the owner. Rabbi Shimon apparently disagrees with the tanna of the mishna on two counts: First, he claims that if the handful of a meal offering was removed not for its own sake it satisfies the obligation of the owner, whereas the mishna teaches that the obligation is not fulfilled. Second, Rabbi Shimon does not differentiate between the meal offering of a sinner or the meal offering of jealousy, and the other types of meal offerings.” (Sefaria.org translation)

Baraita #2 “§ The Gemara cites the baraita that is the basis for the apparent contradiction between the statements of Rabbi Shimon, which was mentioned in the previous discussion: And a Sage raises a contradiction from one statement of Rabbi Shimon and another statement of Rabbi Shimon, as it is taught in a baraita: Rabbi Shimon says that the verse written concerning the meal offering: “It is most sacred, as the sin offering, and as the guilt offering” (Leviticus 6:10), indicates that there are some meal offerings whose halakha is like that of a sin offering, and there are some whose halakha is like that of a guilt offering.

"Rabbi Shimon elaborates: The halakha with regard to the meal offering of a sinner is like that of a sin offering. Therefore, if one removed a handful from it not for its own sake, it is disqualified, just like a sin offering that was slaughtered not for its own sake. Conversely, the halakha with regard to a voluntary meal offering is like that of a guilt offering. Therefore, if one removed a handful from it not for its own sake, it is valid, like a guilt offering that was slaughtered not for its own sake.  

Rabbi Shimon adds: And a voluntary meal offering is like a guilt offering in another aspect as well: Just as a guilt offering is valid but it does not effect acceptance, i.e., it does not satisfy the owner’s obligation, so too, a voluntary meal offering is valid but it does not effect acceptance. Rabbi Shimon here apparently contradicts his ruling that all meal offerings from which a handful was removed not for their sake satisfy the obligation of the owner.” (Sefaria.org translation)

The contradiction is easily summarized. In baraita #1 the minkha offering not for its sake is kosher and the owners get a check mark for fulfilling their obligation. In baraita #2 the minkha offering not for its sake is kosher and the owners don’t get a check mark for fulfilling their obligation.

Three different sages try to resolve this contradiction beginning on daf 2 and continuing on today’s daf 3.

Rabba’s solution: “Rabba said in resolution of this contradiction: This is not difficult. Here, where Rabbi Shimon says that the meal offering satisfies the owner’s obligation, he is referring to a change of sanctity, i.e., it was sacrificed for the sake of another type of meal offering, whereas there, where he says that it does not fulfill the owner’s obligation, he is referring to a change of owner, e.g., the meal offering of Reuven was sacrificed for the sake of Shimon.” (Sefaria.org translation)

Rava’s solution: “The Gemara cites another resolution of the apparent contradiction between the two baraitot that report conflicting opinions of Rabbi Shimon. Rava said: It is not difficult. Here, where Rabbi Shimon says that a meal offering that was sacrificed not for its own sake fulfills the owner’s obligation, he is referring to a case where one removes a handful from a meal offering for the sake of a meal offering. There, where he says that it does not fulfill the owner’s obligation, he is referring to a case where one removes a handful from a meal offering for the sake of a slaughtered offering.” (Sefaria.org translation)

Rav Ashi’s solution: “The Gemara cites a third resolution of the apparent contradiction between the two baraitot that report conflicting opinions of Rabbi Shimon. Rav Ashi said that it is not difficult. Here, where Rabbi Shimon says that the meal offering is fit and fulfills the obligation of the owner, he is referring to a case where one states that he is removing a handful from a pan meal offering for the sake of a deep pan, i.e., he mentions only the vessel and not the offering. There, where it does not fulfill the owner’s obligation, he states that he is removing a handful from a pan meal offering for the sake of a deep-pan meal offering.

“Rav Ashi elaborates: When one bringing a pan meal offering states that he is removing a handful from a pan meal offering for the sake of a deep pan, he has intent only with regard to the type of vessel, and intention with regard to the type of vessel does not disqualify offerings, as he is not sacrificing the vessel, and therefore the owner’s obligation is fulfilled. By contrast, when he states that he is removing a handful from a pan meal offering for the sake of a deep-pan meal offering, he has intent with regard to the type of meal offering, which improper intention does disqualify. Consequently, the owner’s obligation is not fulfilled.” (Sefaria.org translation)

The Gemara concludes why each Sage didn’t agree with the other two.

“§ Three resolutions have been suggested for the apparent contradiction between the statements of Rabbi Shimon in two baraitot concerning whether or not a meal offering whose handful was removed not for its own sake fulfills the obligation of its owner. Rabba’s answer was that there is a difference between intent for another meal offering, in which case the owner fulfills his obligation, as the intent is recognizably improper, and intent for someone else, which is not recognizably improper. The Gemara comments: All the other Sages, i.e., Rava and Rav Ashi, do not say as Rabba did in resolving the contradiction, as they do not accept his reasoning, claiming that on the contrary, the Merciful One disqualifies recognizably false intent.

“The Gemara continues: Rava resolved the contradiction by saying that a meal offering from which a handful was removed for the sake of another meal offering effects acceptance for the owner, as the verse states: “And this is the law of the meal offering” (Leviticus 6:7), indicating that there is one law for all meal offerings, whereas a meal offering from which a handful was removed for the sake of an animal offering does not effect acceptance. Rabba and Rav Ashi do not say as Rava did in resolving the contradiction, as the verse that states: “And this is the law of the meal offering,” does not indicate to them that there should be one law for all meal offerings.  

“The Gemara continues: Rav Ashi resolved the contradiction by saying that when one removes the handful from a meal offering for the sake of a vessel the meal offering effects acceptance for the owner, as intent concerning the vessel itself is inconsequential, whereas when one removes the handful for the sake of another meal offering, the meal offering does not fulfill the owner’s obligation. Rabba and Rava did not say as Rav Ashi did in resolving the contradiction because of the difficulty posed by Rav Aḥa, son of Rava, from a case where one removes the handful of a dry meal offering for the sake of a mixed one. Rabbi Shimon holds that such a meal offering fulfills the owner’s obligation even though his intent concerns the meal offering itself, not the vessel.

“The Gemara notes: The matter that is obvious to Rabba in one way, i.e., that with regard to meal offerings recognizably false intention is disregarded, but when the false intention is not recognizable the offering does not fulfill the owner’s obligation, and that is obvious to Rava in the other way, i.e., that in cases of recognizably false intention the offering should not fulfill the owner’s obligation, is a dilemma for Rav Hoshaya. ” (Sefaria.org translation)