There was a man named Yochanan who served as High Priest for eighty years. Eighty. Longer than most men live, longer than any priest before or since had stood between Israel and the altar. And then, at the very end, he abandoned the tradition of the sages and became a Sadducee.
The Talmud tells this story not to shame him but to warn us. Al ta'amin b'atzmecha — "Believe not in thyself till the day of thy death" (Berakhot 29a). Eighty years of holy service could not guarantee the eighty-first. A man can stand close to the altar his whole life and still, in his final hour, turn away.
The Sadducees (Tzedukim) were followers of Zadok who rejected the oral tradition and clung only to the written Torah, denying the resurrection, denying the world to come, denying the whole unfolding conversation between heaven and earth that the sages had nurtured for generations. To the rabbis, their creed was cold, a body without breath. In Derech Eretz Zuta the warning sharpens: "Learn or inquire nothing of the Sadducees, lest thou be drawn into hell."
The lesson cuts both ways. No righteousness is ever finished, and no teacher is trustworthy simply because he wears the robe.