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.
Pope Paul VI is one who helped confuse what the magisterium is
by introducing an "ecclesiastical magisterium" into the mix and then added theologians, college of bishops, individual bishops, pastors - probably has NO eucharistic ministers and acolytes in there somewhere.These things were not entirely his inventions though, he was merely preaching the same errors that had already by then, "permeated all the manifestations of the Church" as +ABL put it.