CathInfo.com

The link looks to be spot on and agrees with what I said - that the magisterium is always infallible.

As Lee Corso would say: "Not so fast, my friend".  It depends which type you are talking about - there are 3 types of magisteriums and ALL have multiple names.  So confusing!

1.  Solemn/Extraordinary magisterium - when the pope issues a dogmatic, solemn decree (i.e. ex cathedra), as outlined by Vatican I.
Example:  Dogma of the Assumption defined in the 1950s by Pius XII.

2.  Ordinary Infallible magisterium (or 'ordinary and universal') - when the pope teaches "what has always been taught" without using solemn, dogmatic language.
Example:  Humanae Vitae enclycical which re-stated the Church's opposition to birth control and upheld the Natural Law.

-  Cardinal Siri:  "This Encyclical recapitulated the ancient teaching and the habitual teaching of today. This means that we can say that the conditions for the Ordinary irreformable [i.e., infallible - Ed.] Magisterium were met."
-  In the case of the [Ordinary] universal Magisterium, this whole complex is that of the concordant teaching of the bishops in communion with Rome; in the case of the Ordinary pontifical Magisterium [i.e., the pope alone - Ed.], it is the continuity of teaching of the successors of Peter: in other words, it is the "tradition of the Church of Rome," to which Msgr. Gasser appealed at Vatican I (Collana Lacensis, col.404).

My comment:  The ordinary magisterium is only infallible when it is in continuity with the teachings of all the popes (as it says above); when it agrees with the "traditions of the Church of rome".  Ergo, when it teaches "what has always been taught".

3.  Authentic magisterium (or just 'ordinary') - teaching of the pope/bishops/cardinals which has no authoritative language, and does not teach "that which has always been taught", like a simple speech given by the pope to pilgrims in St Peter's square, or his annual 'urbi et orbi' speech, or even a Bishop's conference...or even Vatican II.

This means, in effect, that an "isolated act" of the pope is infallible only in the context of a "dogmatic definition"; outside dogmatic definitions, i.e., in the Ordinary Magisterium, infallibility is guaranteed by the complex of "countless other similar acts of the Holy See," or of a "long succession" of the successors of Peter.


What of Vatican II?
Vatican II did not use its Solemn infallibility because the pope issued no doctrinal statements ex cathedra.  Therefore the following quote applies:
As regards those non-infallible doctrinal decisions given by the pope or by the Roman congregations, there is a strict duty of obedience which obliges us to give an internal assent ...that is prudent and habitually excludes all reasonable doubt, but this assent is legitimized [not by infallibility, but rather] by the high degree of prudence with which the ecclesiastical authority habitually acts in such circuмstances" (entry "Église" in DTC, vol.IV, co1.2209).

Comment:  Since our assent is usually given because the ecclesiastical authorities USUALLY act with prudence, then our assent is legitimized.  If the hierarchy DOES NOT ACT WITH PRUDENCE then our assent has 'reasonable doubts' and it is conditional.  Such is the case with V2.

Since not everything taught by the Ordinary Magisterium is infallible, we must ask what kind of assent we should give to its various decisions. The Christian is required to give the assent of faith to all the doctrinal and moral truths defined by the Church's Magisterium (there were none at V2). He is not required to give the same assent to teaching imparted by the sovereign pontiff that is not imposed on the whole Christian body as a dogma of faith (nothing from V2 is binding). In this case it suffices to give that inner and religious assent which we give to legitimate ecclesiastical authority. (this is why new-rome says we must give 'religious assent' to V2) This is not an absolute assent, because such decrees are not infallible, but only a prudential and conditional assent, since in questions of faith and morals there is a presumption in favor of one's superior (under normal circuмstances, yes, we trust our superiors.  Not in the case of post-V2)....Such prudential assent does not eliminate the possibility of submitting the doctrine to a further examination, if that seems required by the gravity of the question (Nicolas Jung, Le Magistère de L’Èglise, 1935, pp.153,154). 

Comment:  Since V2 did not define, formally teach, bind, or clarify any doctrine, but only proposed PASTORAL APPLICATIONS of doctrine (i.e. how do pastors apply doctrine to the everyday lives of catholics?), therefore its purpose was different from ALL OTHER EcuмENICAL COUNCILS IN THE HISTORY OF THE CHURCH.  For this reason alone (not to mention the novelties contained in it), we have every reason and freedom to pause and consider our 'religious assent' in the face of the doubts and novel approach of our ecclesiastical authorities.

As Lee Corso would say: "Not so fast, my friend".  It depends which type you are talking about - there are 3 types of magisteriums and ALL have multiple names.  So confusing!

Yes, it is confusing, as Lad's link said: "In conclusion we shall excerpt the text of a theologian, whose passing is much to be regretted, who had a very clear grasp of the doctrine we are recalling here, and who knew well that it had been brought into confusion by the New Theologians."

What we know is that V1 mentions 3 separate magisteriums - 1) Divine Magisterium, 2) Ordinary Magisterium and 3) Universal Magisterium. Then theologians and even popes introduced other magisteriums that have or can have, different meanings.

Personally, when I speak of the magisterium, I only speak of V1's - those, being binding, are all always infallible. The magisteriums meantioned by other sources are confusing because it appears that they have more than a single meaning, as such may or may not  be infallible.