The Church has two types of authority:
1. spiritual authority (the teaching authority, called the Magisterium)
2. temporal authority
The doctrines of the Church are Her teachings. Infallible doctrines are called dogmas. But the Church also has disciplines, in addition to doctrines. The doctrines are of the spiritual authority, the disciplines are of the temporal authority.
The temporal authority issues general rules as well as rulings in particular cases. Things that fall under this authority include Canon law (except when the Canon is a direct expression of a teaching on faith or morals), decisions of the Pope in particular cases (not on doctrine), decision of the local Bishop, rules for religious orders, the form of the Mass, etc.
The teachings of the Magisterium are either infallible (no possibility of error) or non-infallible (limited possibility of error). The rules and rulings of the temporal authority are fallible (except for dogmatic facts).
The faithful can, to a limited extent, faithfully dissent from a non-infallible teaching, because these teachings are not guaranteed to be without error. This is termed licit theological dissent. But to be licit, the dissent must be from a non-infallible teaching, not an infallible teaching, and the faithful person who dissents must have a firm basis in the teachings of Tradition, Scripture, Magisterium — not solely in his or her own reasoning.
The faithful can also disagree with, or even disobey (without sin), a rule or ruling of the temporal authority, because these are not teachings that require assent.
For example, a faithful Catholic might disagree with the rule that permits Communion in the hand. A faithful Catholic might disagree with a decision of the Pope on whether a nation or state should have the death penalty, or whether it should be applied in some cases. A faithful Catholic might disagree with a decision of the Pope as to whether a particular war is just or unjust.
A faithful Catholic might disagree with a decision of a local Bishop or of the Holy See as to whether a claimed private revelation is true or false. So if the Medjugorje Commission condemns the apparitions, the faithful can disagree without sin, even if the Pope approves of the decision.