A riddle I will probably never solve.... [and all of the protagonists are in the world of truth].
Gadol "A" was a very fierce opponent of Hagaon "B". I am not certain why. Some say that it is because Hagaon B was caught violating Shabbos as a yeshiva bochur [so went the rumor]. Others believe it had to do with the fact that Gadol A was staunchly opposed to the derech ha-limmud of Hagaon B - or maybe a combination of those and other factors. [Hagaon B was a vocal supporter of "Mizrachi" hashkafos while Gadol A was strongly opposed].
So strong were his feelings, that someone saw the sefer of Hagaon B wrapped up in a towel on the floor of Gadol A's home on Shabbos and Gadol A instructed him not to touch the sefer because it is MUKTZE!!! WHOA!
Anyway, Hagaon had a Yeshiva. A certain Talmid Chochom was hired to teach in this Yeshiva. Gadol A told this Talmid Chochom [who later in life became one of the leaders of the Torah world] to quit his job IMMEDIATELY. The Talmid Chochom had NO PARNASSA and risked starvation if he lost this job. Gadol A told him to quit the job anyway [he obviously felt that it was forbidden to teach there and that Hashem will help this Talmid Chochom find a different parnassa - which He did]. He wanted to go home and tell his wife first but the Gadol insisted that FIRST he quit and then tell his wife, lest his wife dissuade him. That is what he did.
When Hagaon B was told by this Talmid Chochom that he was quitting, he offered him a significant raise in salary. He refused. [What Emunas Chachomim!!! Gadol A remarked that this Talmid Chochom has true Yiras Shomayim].
The riddle: The Rosh Yeshiva of the Yeshiva under the auspices and guidance of Hagaon B was ALSO a devoted talmid of Gadol A. He listened to him even on life and death questions. Yet he served for years as the Rosh Yeshiva under Hagaon B.
Why didn't Gadol A also tell the Rosh Yeshiva to quit?? Or did he and the Rosh Yeshiva didn't listen? Unlikely...
Lesson: Advice given to one person by a Gadol doesn't necessarily apply to a different person.
Another mystery: Just because this Gaon sinned in his youth - does that mean that he continued? We all make mistakes. The gemara says that if a Talmid Chochom sins you have to assume that he did teshuva. Gadol A knew this. So what was the source of his opposition?
Post Script: Not every Gadol felt the same way about Hagaon B as Gadol A. We have letters from them directed to him that were written with great respect.
Lesson: Just because one Rav is against something or somebody, doesn't mean that everyone is. So - we have to stay out of machlokes and let the tzadikim have their machlokos li-shem shomayim. There are many other examples of this.