Formerly, I thought that Councils could teach infallibly, or non-infallibly. And the non-infallible teachings had limited possibility of error.
But Bellarmine says that everything taught by a Council (i.e. on faith and morals) is infallible, as the teaching is of the Pope gathered with the body of Bishops, so the fullness of the Magisterium is teaching. But does this mean that disagreement with any individual point or sentence is heresy? No. I think that the definitions of Councils require the full assent of faith. The lesser points require at least religious assent. Only disagreement with definitions would be heresy. On the other hand, I think the non-definitive teachings of Councils are error-free, but simply not important enough points to cause someone to be cut off from the Church by disagreement.
There are no errors in Vatican II. And I don’t believe the claim that all the documents are “pastoral” and so can be ignored. Nor do I believe that nothing taught by the Council is definitive; LG 25, for example, contains definitions on infallibility of the Magisterium: conciliar infallibility as well as the ordinary universal magisterium. But these passages do require infallible teachings to be definitive, though like the ordinary universal magisterium, definitive does not always mean a definition.
Can we say that any non-definitive teachings of a Council could possibly err? I don’t think so. Councils are error-free in all their teachings. But only definitions and definitive teaching require full assent of faith. Thus, some non-definitive teachings are free from all error, because they are taught by the extraordinary solemn magisterium of a Council.
The other possible position is that everything taught by a Council is definitive. But either way, there would be no errors in the teaching of a Council. This makes sense because Councils are not common, and even a relatively minor error by a Council would do much harm. Also even from a human point of view, it should not be hard for the Pope and Bishops to be able to write on religion and avoid all error. Then with the help of the Spirit, this is guaranteed, supernaturally.
I’ve added below quotes from Dr. Fastiggi’s past comments on this topic:
In De conciliis, Liber II, chapter II, Bellarmine makes this point: “A general Council represents the universal Church, and hence has the consensus of the universal Church; wherefore if the Church cannot err, neither can a legitimate and approved ecumenical Council err.” (Concilium generale repraesentat Ecclesiam universam, et proinde consensus habet Ecclesiam universalis; quare si Ecclesia non potest errare, neque Concilium oecumenicum legitimum, et approbatum potest errare).
“It has been the constant teaching of the Catholic Church from the earliest times that the teachings of the General Councils are infallible.” Ludwig Ott, The Fundamentals of Catholic Dogma Revised and Updated Edition (London: Baronius Press, 2018), p. 321.
St. Robert Bellarmine, at the end of De conciliis, Liber II, chapter IX says “we hold by Catholic faith that legitmate councils confirmed by the Supreme Pontiff cannot err” (ex fide Catholica habeamus concilia legitima a Summo Pontifice confirmata non posse errare).
Bellarmine here is speaking of matters of faith and morals. Some teachings of ecumenical councils, especially on matters of discipline, have been superseded, let go, or revised. Rather than speak of such teachings as errors, I believe it’s more accurate to say these were teachings that were not per se irreformable.