I thought Catholics were required to believe that the magisterium is always right.
If you are interested in understanding Catholic teaching on the role of the Magisterium, the following passage from St. John Paul II may help illuminate how Catholics see the relationship among Christ, the Magisterium and the Christian faithful.
Christians have a great help for the formation of conscience in the Church and her Magisterium. As the Council affirms: "In forming their consciences the Christian faithful must give careful attention to the sacred and certain teaching of the Church. For the Catholic Church is by the will of Christ the teacher of truth. Her charge is to announce and teach authentically that truth which is Christ, and at the same time with her authority to declare and confirm the principles of the moral order which derive from human nature itself ".111 It follows that the authority of the Church, when she pronounces on moral questions, in no way undermines the freedom of conscience of Christians. This is so not only because freedom of conscience is never freedom "from" the truth but always and only freedom "in" the truth, but also because the Magisterium does not bring to the Christian conscience truths which are extraneous to it; rather it brings to light the truths which it ought already to possess, developing them from the starting point of the primordial act of faith. The Church puts herself always and only at the service of conscience, helping it to avoid being tossed to and fro by every wind of doctrine proposed by human deceit (cf. Eph 4:14), and helping it not to swerve from the truth about the good of man, but rather, especially in more difficult questions, to attain the truth with certainty and to abide in it.
http://w2.vatican.va/content/john-paul-ii/en/encyclicals/documents/hf_jp-ii_enc_06081993_veritatis-splendor.html
Canon 753: “While not infallible in their teaching, [Catholic bishops] are the authentic instructors and teachers of the faith for Christ's faithful entrusted to their care. The faithful are bound to adhere, with a religious submission of mind, to this authentic Magisterium of their Bishops.”
An essential aspect of internal Catholic practice therefore requires that the Catholic give religious submission of mind and will to the authentic Magisterium of the Pope and this even if he is not speaking ex cathedra. An essential component of internal Catholic practice is that the Pope's ordinary teaching at a minimum must be religiously submitted to by both intellect and will. (I say at minimum, because if infallible teaching is involved, whether in ordinary or extraordinary mode, the full assent of faith, or theological assent, is required. This is something much more than religious submission.) [http://www.catholic.org/news/hf/faith/story.php?id=49496]