CathInfo.com

The word rule (Latin regula, Gr. kanon) means a standard by which something can be tested, and the rule of faith means something extrinsic to our faith, and serving as its norm or measure. Since faith is Divine and infallible, the rule of faith must be also Divine and infallible; and since faith is supernatural assent to Divine truths upon Divine authority, the ultimate or remote rule of faith must be the truthfulness of God in revealing Himself. But since Divine revelation is contained in the written books and unwritten traditions (Vatican Council, I, ii), the Bible and Divine tradition must be the rule of our faith; since, however, these are only silent witnesses and cannot interpret themselves, they are commonly termed "proximate but inanimate rules of faith". Unless, then, the Bible and tradition are to be profitless, we must look for some proximate rule which shall be animate or living. [Goes on to demonstrate that this proximate animate/living rule is the Church/Magisterium]

I have never seen a quote from the Magisterium itself which says that it is not infallible. There are many quotes that describe itself in different ways (authentic, living, ordinary, supreme etc...) and they all say it is infallible. Maybe there are theologians that say the Magisterium can err, I haven't seen any, but there is nothing from the Popes that say this.
The Magisterium is the teaching office of the Church, how could it err in anything?

What you are alluding to is correct, the magisterium is always infallible.