Gemara

29b (Mishnah - R. Yehudah): If Reuven is suspected to sell Terumah and say that it is Chulin, one may not buy from him even water or salt;

R. Shimon forbids (only) anything to which Terumah applies.

"Anything" includes fish innards, for often olive oil is added.

A certain butcher was suspected of selling forbidden Chelev (fat) of the kidney and saying that it is permitted Chelev. Rava fined him, and forbade him to sell even nuts.

Rav Papa: Your fine is according to R. Yehudah. If so, you should forbid even water and salt!

Rava: No, it is according to R. Shimon. The fine applies only to the Isur;

Children crave nuts. Reuven gave nuts to butchers' children, in exchange for which they bring to him forbidden Chelev (which their fathers had little use for anyway), and he would sell it.

(Mishnah): One who is suspected about Ma'aser is not suspected about Shemitah.

(Rabah bar bar Chanah): Our Stam Mishnah is like R. Akiva, but Chachamim say that one who is suspected about Ma'aser is suspected about Shemitah.

These Chachamim hold like R. Meir, who says that one who is suspected about one matter is suspected about every Mitzvah.

35b - Question (Rav Papa): R. Meir says that one who is suspected about a matter cannot testify about or judge it, and that one who is suspected about one matter is suspected about everything in Torah;

Since Kohanim are suspected about Mumim, they should be disqualified from judging. However, it says "v'Al Pihem Yihyeh Kol Riv v'Chol Naga"!

Answer (Abaye): R. Meir suspects a Kohen concerning Mumim. He is not established to lie about it.

Chulin 5b (Beraisa): "From commoners" excludes a Mumar (he does not bring a Chatas for a sin b'Shogeg);

R. Shimon ben Yosi says, one who sinned b'Shogeg, and would not have sinned knowingly, brings a Korban. One who would have sinned anyway does not bring a Korban.

(Rav Hamnuna): They argue about one who wantonly eats Chelev, and mistakenly consumed blood. (The first Tana exempts him from a Korban. R. Shimon obligates him.)

6b (Mishnah): One who gives to his inn-keeper (dough to bake) must Ma'aser the dough he gives to her and the bread he receives, because she is suspected to switch it.

Inference: We are concerned lest an ignoramus switch one's food.

Rejection: There, she rationalizes her action. This Talmid should eat fresh, warm bread, and I will eat cold bread!

Rishonim

Rambam (Hilchos Ma'achalos Asuros 8:10): If one bought meat and sent it with an Am ha'Aretz, he is believed about it. Even though he is not established to be Kosher, we are not concerned lest he switch. Even slaves of Yisrael are believed about this, but not Nochrim.

Magid Mishneh: The Gemara (Chulin 6b) concludes that we are not concerned for switching. One may deposit tithed produce with an Am ha'Aretz, for he is not suspected to steal. The Rashba says that one may give hemp threads to a tailor (and ask him to fix his wool garment) if the tailor is suspected of using linen threads. However, there is room to be stringent, since it is harder to sew with hemp (than with linen), and perhaps the tailor will switch. Surely one may not ask the tailor to sew with the tailor's own hemp!

Rashba (Toras ha'Bayis 2:2 28a, brought in Beis Yosef YD Sof Siman 119): One who is suspected to eat things that people are not normally lenient about is suspected to switch. He is like a Nochri. Anything that pertains to a Torah Isur, one may deposit with him only with two seals.

Magid Mishneh (ibid.): He learns from Avodah Zarah 31a, which says that one seal does not suffice for Isurei Torah, and Rabanan establish this to discuss a Yisrael who is suspected.

Beis Yosef (ibid.): It seems that the Tur disagrees. He requires two seals only for Nochrim.

Rashba (ibid.): We say that one who is suspected is not suspected to switch only regarding one suspected to eat things that people are normally lenient about, e.g. Amei ha'Aretz eat Demai, and people are lenient about cheese of Nochrim.

Tosfos (36a DH Eimar): Abaye answered that R. Meir only suspects a Kohen concerning Mumim. This connotes that the Halachah does not follow R. Meir. Rather, even if one is Muchzak to transgress one matter, he is not suspected about the entire Torah. In Sanhedrin (27a), Abaye says that one who eats Nevelos to anger Hash-m is Pasul for testimony, and he establishes this like R. Meir. This is one of the six Halachos in which the Halachah follows Abaye against Rava! That is only regarding testimony, for a verse forbids testimony of a Rasha. He is not suspected about other matters. R. Meir disqualifies an Ed Zomem (he testified about something that he was not present to see) from testifying. He does not exclude (one who is suspected about) other matters. He mentioned Ed Zomem to show that he argues with R. Yosi, who holds that an Ed Zomem about a monetary case is believed to testify about capital cases.

Poskim

Shulchan Aruch (YD 119:4): One who is suspected about one matter is not suspected about others matters.

Beis Yosef (DH ha'Chashud): It is known that when R. Yehudah and R. Shimon argue, the Halachah follows R. Shimon.

Rebuttal (Bach 3): This resulted from an errant text. Eruvin 46b explicitly says that when they argue, the Halachah follows R. Yehudah. However, since Rava held like R. Shimon, this shows that in this case the Halachah follows R. Shimon.

Gra (Likut): Even though R. Meir argues, the Halachah does not follow R. Meir, like Tosfos says.

Shulchan Aruch (ibid.): However, he is suspected about everything needed for that matter. E.g. if one was suspected to sell Chelev in place of Shumen, and he used to give nuts to children to induce them to come to buy, we fine him and forbid him to sell even nuts.

Question (Bach 3): The Gemara explicitly says that he would give nuts to butchers' children, in exchange for which they bring to him forbidden Chelev, and he would sell it!

Taz (7): Why did the Tur say that he used the nuts to encourage the children to buy from him? One can ask why Rava did not answer like the Tur says. Do not say that Rava answered exactly like the case was. If so, why did Rava say that children desire nuts? Even if this were not normally true, since this time the nuts were used to entice, he forbade selling them! Rather, we must say two things. One is that they (the nuts) were needed for the forbidden sale. The other is that we fine not to sell what is needed (for the forbidden sale) only if there is suspicion about it itself. The Tur taught how the nuts were needed for selling the Isur. The Gemara agrees; it taught how there is suspicion regarding the nuts themselves, without this sale. Since children crave nuts, the nuts can entice them to steal from their fathers. We suspect lest they already stole, therefore we fine for the future.

Question: What was the Tur's source to say that nuts helped accustom children to buy from him?

Answer (Taz): The Gemara said 'Stam children (crave nuts...)' This shows that it is not a mere Safek. The Prishah says that even if we did not see an Isur, we are concerned lest an Isur occur. This is wrong. The Tur wrote 'he used to accustom them.'

Bach (2): We discuss things that many are lenient about them. Even so, one who is suspected about one such matter is not suspected about other such matters. Many are lenient about Shevi'is, Ma'aser and Taharos. One who is suspected about one of them is not suspected about the others, but he is suspected about everything needed for that one, even if it is more stringent. A proof is from the butcher. They forbade him to sell even nuts. All the more so he is not believed about Isurim that many are lenient about, even if people consider it more stringent than what he is suspected about. The butcher was suspected about Chelev, which not many are lenient about it. Therefore he was suspected about all Isurim. R. Yehudah and R. Shimon argue only about a fine not to sell water and salt. At the end of this Siman, the Beis Yosef brings like this from the Rashba. He is suspected even to switch. The Beis Yosef says so in Siman 65. Many are not careful about cheese of Nochrim, therefore one who is suspected about this is not suspected about switching. Other Isurim that people are careful about, even though they are only mid'Rabanan, one who is suspected about them is suspected about switching. Why didn't the Tur explain this more? Also the Rashba wrote Stam (without giving enough details).

Shach (11): The Bach holds that a butcher suspected about Chelev is suspected about everything (even Isurim more stringent than Chelev), for there are not many who are lenient about this. He learns from the Rashba (above). This is wrong. Even the Rashba says only that one who is suspected about something that not many are lenient about it, he is suspected to switch it. Tana'im argue about whether a Mumar to eat Chelev brings a Korban for blood (if he ate it b'Shogeg, i.e. if we say that had he known he would not have eaten it). Tosfos (DH me'Am) explicitly says that he is not suspected about another matter.

Taz (6): The Shulchan Aruch teaches that he is suspected about it, i.e. he may not sell even it by itself.