Law.com has a National Law Journal article that describes the ethics, and the dangers, of metadata. The discussion isn’t so much on the evidence side, as on the side of correspondence between attorneys; to wit:
As the District of Columbia Bar recently explained:
“A lawyer who is preparing a document may electronically circulate the document in draft form among other lawyers in the firm for their review and comment. The other lawyers may insert their suggested revisions and other comments, some of which might address the strengths and weaknesses of the client’s position. If the final version of the document is electronically transmitted to opposing counsel, it may be possible for opposing counsel to discover the comments.
“The sender of the document may not be aware of the metadata embedded within the document, or that it remains in the electronic document despite the sender’s good-faith belief that it was ‘deleted.’”
The article talks of instances where ignorance of the presence of metadata proved damaging:
Examples of unintended releases, he offered, include one firm that posted a Word document online and, with two knowledgeable clicks, a savvy viewer was able to discover that the client initially intended to sue someone other than the named defendant. In another case, a motion in a national security lawsuit was posted with what the lawyers thought had been redacted sensitive information. That information, said Hricik, was easily uncovered by metadata-savvy viewers.
Some bar associations prohibit the mining of data by from documents prepared by other attorneys, likely on grounds associated with the work product privilege. Attorneys often warn their clients that “ignorance of the law is no defense.” Now, attorneys are being warned the same about metadata.