Gemara

41b (Beraisa - R. Nasan): When Chachamim told Agripas 'you are our brother (i.e. fit to be king)', the haters of (a euphemism) Yisrael deserved to be destroyed for flattering him.

(R. Shimon ben Chalafta): From the day that flattery became rampant, judgments became distorted, deeds became spoiled, and no one can say 'my deeds are greater than yours.' 3. (R. Elazar): A flatterer brings anger to the world - "flatterers at heart put anger." His prayer is not heard - "they will not be saved when afflicted." He goes to Gehinom. A Tzibur with flatterers is despised like a Nidah; in the end, it is exiled.

If Reuven flatters Levi, he will eventually fall into Levi's hands, or into the hands of Levi's sons, or grandsons. We find that Yirmeyahu supported Chananyah's false prophecy; Chananyah's grandson later handed him over to the Babylonians.

(R. Yirmeyah bar Aba): Four categories do not receive the Divine Presence: scoffers; flatterers; liars; and those who speak Lashon ha'Ra.

Chulin 94a (Beraisa - R. Meir): If one opens barrels of wine in honor of a guest, and he already agreed to sell them to a grocer, he must inform the guest. If this is done to honor the guest in front of others, it is permitted.

Question: Ula visited Rav Yehudah, and Rav Yehudah opened barrels that he had sold to a grocer!

Answers: Rav Yehudah informed Ula. Alternatively, Rav Yehudah loved Ula so much that he would have opened them even if they were not sold.

(Beraisa): A person should not go to a mourner's house with a half-empty flask, nor should he fill it with water, for he deceives him (he thinks that it is full of wine). One may do this to honor the mourner in front of many people.

94b: Mar Zutra bar Rav Nachman encountered Rav Safra and Rava on the road. He said that they did not need to come so far to greet him!

Rav Safra: Really, we didn't know that you were coming. Had we known, we would have come further to greet you!

Rava (to Rav Safra): Why did you say that? It will make him feel bad!

Rav Safra: If I did not say so, we would have tricked him!

Rava: No, he tricked himself.

Rishonim

The Rif (Chulin 33a-b) brings R. Meir's Beraisa and the episode with Mar Zutra.

Question (Tosfos Chulin 94b DH v'Ha): Why did Rav Safra say that had they not told Mar Zutra that they didn't come to greet him, they would have tricked him? They would have done so anyway, so this is not deception, like we said about Rav Yehudah and Ula!

Answer #1 (Tosfos): Rav Yehudah truly opened for Ula; he would have done so even if they were not sold. Rav Safra and Rava did not come to greet Mar Zutra.

Rambam (Hilchos De'os 2:6): One may not speak words of flattery or sweet-talk. One may not speak one way and think differently in the heart. His inside must be like his outside. One may not open a barrel that must be opened in order to sell it as if he does so to honor a guest. Even one word of sweet-talk is forbidden.

Question (Kesef Mishneh and Beis Yosef CM 228 DH v'Af Lignov): Why did the Rambam and Tur (and Rif and Rosh) omit the Heter for one wants to show the dearness of his friend?

Answer #1 (Beis Yosef): This is obvious; it need not be taught.

Answer #2 (also to Question 2 - Lechem Mishneh): Tosfos permits Moshe to do an act to honor Levi if this is truly his intent, even if Levi will err and think that Moshe will lose money for this, as long as Moshe would do so even if it entailed a loss. The Rambam and Tur reject Tosfos' answer, for Rava and Rav Safra do not permit in order to show dearness. Rather, the Halachah follows the first answer (Rav Yehudah informed Ula).

Answer #3 (Divrei Chamudos on Rosh, Chulin 7:84): The Rif and Rosh omit this because they are stringent not to rely on the second answer.

Answer #4 (Mar'eh ha'Panim, Avodah Zarah 3b DH Tani): The Rambam forbids only deception. He need not say that what is for honor is permitted.

Rosh (Chulin 7:18): It is Geneivas Da'as to open a barrel for Levi Stam, without telling him that it was sold to a grocer, for Levi will not know that it was sold. This is unlike the case with Mar Zutra, he deceived himself. Mar Zutra should have realized that they did not come to greet him. How would they know that he was coming?! Rashi forbids opening a barrel only when he says that he opened it to honor him. '(It is permitted) only if he informed him' connotes otherwise. Also, what was the question from Rav Yehudah? Surely he did not lie (and tell Ula that he opened them for him)!

Note: What is the latter question? Rashi (DH v'Lo) explains that usually, opening a barrel causes a loss, for the wine will not last long. If it was already sold to a grocer, there is no loss, it will be sold quickly. Rav Yehudah did not open the barrel until Ula came. He truly opened it to honor Ula, just he did not say that he will not lose from this. Divrei Chamudos (83) says that a sold barrel was surely opened, for a grocer would not buy it without tasting it. Rashi (Bava Metzi'a 23b DH b'Rashum) explains that in Shevat or Nisan, sellers open many barrels and grocers taste the wine before buying. What will not be sold immediately (to consumers) is resealed, and will not spoil quickly (until it is opened again). Perhaps Rav Yehudah reopened a barrel that had been opened, sold and sealed. The grocer was ready to sell it to consumers, so there would be no loss!

Poskim

Shulchan Aruch (CM 228:6): One may not deceive people. One may not open barrels opened for a grocer, and Levi thinks that they were opened for him. Rather, he must tell Levi that he did not open them for him. If it is something that Levi should realize that (perhaps) it is not done for him, one need not inform him. E.g. one encounters Levi on the road, and Levi thinks that you came to greet him to honor him.

SMA (9): The Tur forbids making it seem as if one opens for the sake of Levi. We do not say that Levi have seen that the barrel was already open. Perhaps his text in the Gemara said 'opened', not sold.