Wood v. Milyard, 132 S.Ct. 1826 (April 24, 2012) (state deliberately waived the statute of limitations defense, where it twice informed the district court that it would "not challenge, but [was] not conceding," the timeliness of a prisoner's federal petition for habeas corpus; courts of appeals, like district courts, have the authority--though not the obligation--to raise a forfeited timeliness defense on their own initiative in exceptional cases).