Rabbi Shlomo Yitzhaki, better known today as Rashi, was easily the most prolific commentator on the Tanach and Talmud in history in the sense that he commentated on most of the Talmud and, I believe, all or most of Tanach (although his comments are typically short and sweet). His interpretations of Biblical verses and the discussions of Talmud are so important to Jewish thought that they are still taught, nearly 1,000 years later, to students as young as 3rd or 4th grade and older students will typically look to Rashi's interpretations before other commentators.
What is fascinating though is that the Tosafists -- later Biblical and Talmudic commentators who saw their comments as supplemental to Rashi's -- and who include Rashi's own grandchildren, Rabbeinu Tam and Rashbam, often openly disagreed with Rashi. Haim Perlmuter, in Tools for Tosafos, writes at p 14 that "with all the honor they had for Rashi, they did not hesitate to disagree with him." While as academics, I applaud their critical independence, but given the tradition of passing on the tradition from teacher to student, the general rule against contradicting one's teacher, and the fact that Rashi was the grandfather of Rebbeinu Tam and Rashbam, I would think that they would be more deferential to their grandfather. How did they come to have such different views from their grandfather on so many topics? Was he not their teacher or their teachers' teacher?
Is it OK to poke holes in one's rebbe's books after he has passed away and cannot defend his work? Perlmutter's book, at page 84, offers some possible answers, but I'd like to see more.