Be Careful Basing Your IPR On Previously Considered Prior Art, PTAB Litigation Blog
It is not always possible for a party seeking to challenge a patent in an IPR to find prior art patents or printed publications that the USPTO has not already considered. Often the best available prior art comes from the references cited to (or by) the Office. It is fairly common, therefore, for an IPR petitioner to rely on prior art that was of record during prosecution of the challenged patent (or that was of record during a different challenge such as ex parte reexamination or another IPR).
Read the full article at ptablitgationblog.com.