For the purposes of this title, an allegation of error is finally litigated when: an appellate court of the State decides on the merits of the allegation: on direct appeal; or on any consideration of an application for leave to appeal filed under § 7-109 of this subtitle; or a court of original jurisdiction, after a full and fair hearing, decides on the merits of the allegation in a petition for a writ of habeas corpus or a writ of error coram nobis, unless the decision on the merits of the petition is clearly erroneous. Except as provided in subparagraph (ii) of this paragraph, an allegation of error is waived when a petitioner could have made but intelligently and knowingly failed to make the allegation: before trial; at trial; on direct appeal, whether or not the petitioner took an appeal; in an application for leave to appeal a conviction based on a guilty plea; in a habeas corpus or coram nobis proceeding began by the petitioner; in a prior petition under this subtitle; or in any other proceeding that the petitioner began. Failure to make an allegation of error shall be excused if special circumstances exist. The petitioner has the burden of proving that special circumstances exist. When a petitioner could have made an allegation of error at a proceeding set forth in paragraph (1)(i) of this subsection but did not make an allegation of error, there is a rebuttable presumption that the petitioner intelligently and knowingly failed to make the allegation. This subsection applies after a decision on the merits of an allegation of error or after a proceeding in which an allegation of error may have been waived. Notwithstanding any other provision of this title, an allegation of error may not be considered to have been finally litigated or waived under this title if a court whose decisions are binding on the lower courts of the State holds that: the Constitution of the United States or the Maryland Constitution imposes on State criminal proceedings a procedural or substantive standard not previously recognized; and the standard is intended to be applied retrospectively and would thereby affect the validity of the petitioner’s conviction or sentence.