Additionally, one could have seen and misinterpreted an abbreviation, writing a new word altogether. Rabeinu Gershom edited the correct text, deleting the erroneous additions, and wrote the correct text in his own handwriting.
This was not a simple job of editing, but rather a job which could only be done by someone with encyclopedic knowledge of the Talmud. Someone who could ascertain an error based on clear texts, often in other tractates of the Talmud. The Talmud often deals with a certain topic in many different tractates. Only someone who is extremely familiar with all of those sources, and is aware of the logical depth of each passage, can judge whether or not a certain word in one of these discussions is erroneous.
Clearly, this method of thoroughly learning the Talmud so that one could be capable of finding the correct text, down to a word and even to a letter, was handed down to Rashi. Time and time again we see that Rashi quotes more than one text and clarifies which is more accurate. We also find that he noted words which were commonly added in certain texts, and said they should not be added. This method of learning was clearly gained from Rabeinu Gershom’s exacting approach towards Torah.
Rabeinu Gershom also wrote a commentary on the Talmud which explained the Gemara, and which sometimes included mention of its Halachic ramifications. Though it is said that he wrote a commentary on the entirety of the Talmud, only commentaries on certain tractates are extant. Rashi mentions that “we live from his (Rabeinu Gershom’s) mouth”.
It is therefore no surprise that Rashi emulated Rabeinu Gershom and his students, Rashi’s teachers, by composing his own commentary whose purpose is to explain the Talmud.
Rashi’s format for this work was innovative. He broke down nearly every word and thought, instead of explaining broad ideas. The main thrust of his commentary was to explain the Gemara, and not to be a Halachic codifier. However, we see that his explanations are also often used to determine Halachah.
For more on the the uniqueness of Rashi’s commentary, and the difference between explaining the Gemara versus clarifying the Halachah, see chapter 6.