The Magisterium teaches infallibly in any of three ways:
1. solemn definitions of the Pope (papal infallibility)
2. solemn definitions of Ecumenical Councils (conciliar infallibility)
3. the ordinary and Universal Magisterium (when the body of Bishops dispersed through the world, in union with the Pope, authentically teaching on matters of faith and morals, are in agreement on one position as definitively to be held by all the faithful; cf. Lumen Gentium, n. 25)
Infallible teachings are certainly without error; their freedom from error is guaranteed by the Holy Spirit teaching through the Pope and the body of Bishops.
All other teachings of the Magisterium are non-infallible, non-irreformable, and subject to a limited possibility of error. The errors possible are more than merely trivial errors. But there errors also cannot extend so as the lead the faithful away from the path of salvation.
Infallible teachings require the full assent of faith (theological assent), because they are certainly without error, and are necessary to salvation. Non-infallible teachings require a different type and a lesser degree of assent, called the religious submission of will and intellect (religious assent). But since the non-infallible teachings allow for a limited possibility of error, they also allow for a limited possibility of faithful dissent; the full assent of faith is not required.
The First Vatican Council infallibly taught that the teaching of the Pope is only infallible when certain criteria are met. Whoever claims that the teaching of the Pope is infallible under a lesser or different set of criteria, or that the teaching of the Pope is always infallible, is rejecting the infallible teaching of the First Vatican Council and has fallen into heresy. See this related article:
Is it a Heresy to Believe that the Ordinary Magisterium is Infallible?
So because the teachings of the Magisterium are not always infallible, dissent from non-infallible teachings is not always sin or unfaithfulness. Faithful dissent is possible, to a limited extent.
The U.S. Bishops have taught that there are norms for licit dissent, in other words, that within certain limits the faithful may dissent from some non-infallible teachings. This teaching on the norms of licit dissent is a teaching of the Magisterium.
“Norms of licit theological dissent
“There exist in the Church a lawful freedom of inquiry and of thought and also general norms of licit dissent…. The expression of theological dissent from the magisterium is in order only if the reasons are serious and well-founded, if the manner of the dissent does not question or impugn the teaching authority of the Church and is such as not to give scandal.”
It is not the case that all Bishops are always in agreement with one another in their exercise of the Magisterium. If they disagree in their teaching, someone must be mistaken. The faithful may dissent from the teaching of some Bishops, in order to adhere to the teachings of other Bishops, or of Tradition, Scripture, Magisterium in general.
Neither is it the case that all Bishops must agree with every non-infallible teaching of each Pope. The Bishops are the successors to the Apostles; they too have a teaching ministry from God by virtue of ordination to the episcopal degree. They may disagree with the non-infallible teaching of the Pope, to a limited extent, faithfully and without sin.
And it is a perhaps a misguided sense of loyalty to the Magisterium that motivates some Catholics to speak as if every magisterial teaching is infallible. But worse still is the assertion that we should believe all non-infallible teachings, on every point, with no regard for whether or not those teachings are true on every point. God is Truth, and this is the main reason that the Catholic faithful can licitly and faithfully dissent from some non-infallible teachings.