Tosafoth were the sons-in-law, grandchildren, and students of Rashi (and their students), and "they" wrote a commentary on the Talmud that is often at odds with Rashi's earlier commentary. Sometimes ...
In the margins of the g'mara are many notations indicating changes that various rabbis (Bach, G'ra, Tzon Kadashim, Shita M'kubetzes, etc.) made in the wording of the g'mara, seemingly to resolve ...
We don't have (and more's the pity!) R' Yehudah Hanassi's autograph copy of the Mishnah (well, okay, it's a controversial issue whether he actually wrote it or only arranged it orally), or Ravina and ...