The Gemara cites a story at the bottom of the amud where a group of people saw someone coming near them, and decided to ask him questions that he would not be able to answer and thereby push him off.
Why is this acceptable; it appears that they were asking only for the purpose of stumping him, and perhaps embarrassing him?
Gershon Dubin
1. The Chafetz Chaim, at the end of his Sefer on the laws of Lashon ha'Ra, prints a responsum of the Chavos Ya'ir (#152) who asks a question similar to yours, on the general topic of why we find sometimes in the Gemara very sharp comments exchanged between the different Chachamim. He asks that since we know that "the words of the wise are heard when they are said calmly" and the Mishnah tells us that the honor of our friend should be as dear to us as our own honor, why do we sometimes find provocative and seemingly uncaring expressions stated by the holy Sages in the Gemara?
2. The Chavos Ya'ir cites the Gemara in Bava Kama (105b) where Rava called Rav Amram "Tadora," which Rashi explains to mean a person without a heart. The Chavos Ya'ir writes that because the Amora'im were very great and they believed, in their humility, that their colleagues were greater than they, they consequently were not particular about such expressions. (See the Chavos Ya'ir there for other approaches.)
3. In addition, the Gemara in Bava Basra (21a) states that "Kin'as Sofrim Tarbeh Chochmah" -- "the jealousy of scholars will increase wisdom." A competitive atmosphere is conducive to advancement in Torah study. Therefore, we can understand why unfavorable commments were made about Rav Yosef the son of Rav Menashya of D'vil -- since anyway he would not take these comments to heart and, in addition, the competition was positive for encouraging higher levels of achievement in Torah study.
4. In fact, it is easier to understand why they asked Rav Menashya questions with the intention of tripping him up. We find elsewhere in the Gemara that the Sages were asked difficult questions with the intention of finding out if they were great Talmidei Chachamim. See Bava Basra (22a) where the Resh Galuta told Rava to ask Rav Dimi a difficult question to see if he was a sufficient Talmid Chacham to be given privileges in the marketplace. The phrase used by the Gemara there is "Tahah Lei b'Kankanei." Rashi explains that this means, "Smell the wine in his barrel and determine whether it is wine or vinegar." In other words, check him out by asking him Halachic questions in order to determine whether he is a Talmid Chacham.
5. This Torah examination is, of course, essential in order to know whether a person can be relied upon or not to rule in Halachic matters.
Kol Tuv,
Dovid Bloom