Great question. This actually rises to the level of a professional responsibility rules issue in many jurisdictions. Take, for example, this recent-ish opinion from the California Bar's ethics committee:
The best practice is clearly to run the diff from your own last revision, but that may not be available to the attorney. Sometimes clients forward opposing markups of drafts that were prepared without an attorney, perhaps based on a form. Business people may tweak a draft between attorney revisions, as they closed on deal points. It might be worthwhile to go over and attribute each unrecognized change, but not always. Clients may vouch that all changes needing review are in Track Changes. Sometimes speed matters more. Between repeat-player attorneys, or when reputations for ethics and rigor otherwise precede each side, the risk of this kind of thing approaches zero, and everybody saves money.
I have seen things "slipped in", often inadvertently (turned Track Changes off, diffed the wrong files), but sometimes maliciously. I have also done very long days of nothing but verifying execution copies of documents as they were closed out in the run up to big-dollar closings and low-dollar slug-fests. When the transaction is large or important enough, attorneys will handle exchanging drafts across sides and enforce more rigorous change control.
http://ethics.calbar.ca.gov/Portals/9/documents/Opinions/CAL...
The best practice is clearly to run the diff from your own last revision, but that may not be available to the attorney. Sometimes clients forward opposing markups of drafts that were prepared without an attorney, perhaps based on a form. Business people may tweak a draft between attorney revisions, as they closed on deal points. It might be worthwhile to go over and attribute each unrecognized change, but not always. Clients may vouch that all changes needing review are in Track Changes. Sometimes speed matters more. Between repeat-player attorneys, or when reputations for ethics and rigor otherwise precede each side, the risk of this kind of thing approaches zero, and everybody saves money.
I have seen things "slipped in", often inadvertently (turned Track Changes off, diffed the wrong files), but sometimes maliciously. I have also done very long days of nothing but verifying execution copies of documents as they were closed out in the run up to big-dollar closings and low-dollar slug-fests. When the transaction is large or important enough, attorneys will handle exchanging drafts across sides and enforce more rigorous change control.