Well, it definitely wasn't added in a later edit, or at least, not that I'm aware of, though I do have a tendency to write my comments out in bits submitted piece-by-piece. Even so, I wouldn't worry about it, I tend to miss whole blocks of text with alarming regularity while reading through stacks of pdfs and when comparing notes with colleagues we always wonder if we've been reading the same documents (they have the same problem...). Reading in parallel is our way of trying to ensure we don't miss anything and unfortunately it is not a luxury.

Often the effects are more subtle, reading what you think something said rather than what it actually said, or missing a negation or some sub-clause that materially alters the meaning of a sentence.

Even in proofreading we find stuff that is so dead obvious it is embarrassing. On the whole visual input for data is rather unreliable, even when reading stuff you wrote yourself, which I find the most surprising bit of all.

Studying this is interesting, and to some extent important to us due to the nature of our business, missing critical info supplied by a party we are looking at could cause real problems so we have tried to build a process to minimize the incidence of such faults, even so I'm 100% sure that with every job we will always miss something, and I live in perpetual fear of that something being something important.