You can’t waive jurisdictional arguments, except when you can waive them.

At oral argument, the appellant in a technical dispute about the appointment of arbitrators “argued for the first time that ‘if maritime jurisdiction applies, then . . . there is appellate jurisdiction over the appeal.'” The Fifth Circuit observed: “We do not usually allow parties to raise a new argument for the first time at oral argument. . . . Of course, an argument that this court lacks jurisdiction cannot be waived, but here the argument is that the court has jurisdiction, a matter the appellant is required to prove.” Bordelon Marine, LLC v. Bibby Subsea ROV, LLC, No. 16-30847 (April 14, 2017, unpublished).