Validity? 13th Centural Roman Pontifical

I was always under the impression the Pope couldn't promulgate a law the could invalidate a sacrament. But I was reading an old Angelus Magazine and it had this in it "The Roman Pontifical of the 13th century contained an even more surprising error: it affirmed that the consecration of wine into the blood of Christ could be effected even without the words of consecration by mere contact of the wine with a consecrated host." The author, a priest, continued in answering another question: "These errors are possible because in approving these rubrics, the Holy See did not intend to give them the value of dogmatic definitions. That was clear to all. (Theologians were discussing the question of the matter of the sacrament of holy orders until the time of Pius XII; they did not consider the rubrics as sufficient to decide the question.)" And in response to another question regarding the subject said "These examples clearly show that the Holy See does not always engage its infallibility in liturgical matters; in order to ascertain to what extent infallibility is engaged, one must carefully consider the nature, the essential content, the circumstances, and the degree of authority of official decisions." One can read the questions and answers here: http://www.angelusonline.org/index.php?section=articles&subsection=show_article&article_id=2799. My question is how can we know for certain when a certain law is infallible or not?