Since writing began people have been making notes in the margins. We know for sure that this happened with scripture text because we have examples of it on some of the ancient manuscripts. I have several study bibles where I have made notes to my self in the margins so I am as guilty as those ancient scribes.
One example of this on an ancient copy of Luke 3:23 in the genealogy of Jesus. A scribe or scholar came to the part about Jesus being the son of Joseph. Knowing that Jesus was the son of God he wrote in the margin beside that verse the words, “As was thought” as simply a marginal note. Perhaps years later another scribe was given the task of making a copy of that manuscript. He came to the place where the marginal note had been placed and thought that the previous scribe had left it out of the text and wrote it in the margin. He promptly included the marginal note in the text of the new copy. There it stayed every time a new copy was made.
It is still there however new translations often put it in brackets to indicate that it was not in the original text. There are a lot of examples of this same kind of scribal error.