As an alien has the power to challenge the knowingness of waiver and petition for an extension, overstaying a voluntary departure order exhausts the benefit of permitted departure and allows the government to finalize the alternative order of removal, with all waivers of the voluntary departure grant intact.
(Again, all this is quick work. Not legal advice. Don't rely.)
http://media.ca8.uscourts.gov/opndir/18/06/163506P.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.