Previously, I had thought there might have been prohibitions against spies and law enforcement officers from going undercover as journalists, e.g. because this puts real journalists in greater danger of being prosecuted/dealt with as spies and because folks who might otherwise blow a whistle on unethical practices might reasonably expect their press contact to be an undercover agent really seeking to out the person and help ensure the information remained hidden.
However, the Department of Justice Office of the Inspector General recently ruled that FBI agents may impersonate journalists while conducting undercover investigations.
According to that linked article, FBI Director James Comey has previously called the practice "lawful and, in a rare case, appropriate." He claims that the "use of such techniques is subject to close oversight, both internally and by the courts that review our work."
What are the standards applied in this oversight, in theory or in practice? Are there known examples of requests that failed even the internal oversight process, or clear differentiators between instances where impersonating a journalist is ruled OK vs. those where it's ruled not OK?