In a number of jurisdictions, antitrust authorities may challenge consummated mergers even when the parties were not required to report those deals in the first instance or where the parties reported the deals, but the authorities did not initiate challenges after their pre-closing review. This article identifies some examples of merger challenges in these circumstances and other antitrust risks that arise with nonreportable mergers, and discusses lessons parties should draw from the antitrust authorities’ enforcement practices.