Legal problems arose during the Council that have been pointed out by authors such as Romano Amerio (Iota Unum) and Ralph M. Wiltgen (The Rhine Flows into the Tiber). Did it involve substantive canonical procedure such as to render the legality of the procedings at least suspect? In civil jurisprudence, an appellate court examines a case in order to ascertain the rectitude lower court's trial. When it finds an error that caused harm to one of the parties, the case is remanded for correction. In the case of the legal framework of a Council, would it be possible for a legal problem of sufficient import to render it invalid (canonically speaking)?
A key principle to note before we reply, "Yes, the Pope promulgated it" is that the legislator judges the law when he acts as such, but when he is not legislating, he is subject to the law. The Church is not a lawless society, with Popes and Bishops doing as they please because they are in authority. Many actions, if not done according to the law, are to be considered invalid.
Now, I wonder if the legal issues were such that the conciliar texts, decrees, constitutions, etc. were rendered legally invalid. It's an interesting question that should be investigated.