Bahá’ís give credence to the interpretations of ‘Abdu’l-Bahá and Shoghi Effendi and consider them to be authentic and infallible, while any statement of an individual that is contrary to the spirit and form of the authorized interpretations of the Word of God is not acceptable. Furthermore, no pronouncements by the learned on various aspects of the Faith are considered valid unless sanctioned by the Universal House of Justice.
(Adib Taherzadeh, The Child of the Covenant, p. 386)

Everything the House of Justice decides has to be obeyed. This obedience to the two supreme institutions, the Guardian and the Universal House of Justice, has been emphasized by ‘Abdu’l-Bahá in unequivocal language: Whoso obeyeth him not, neither obeyeth them, hath not obeyed God; whoso rebelleth against him and against them hath rebelled against God; whoso opposeth him hath opposed God; whoso contendeth with them hath contended with God... May the wrath, the fierce indignation, the vengeance of God rest upon him! [Will and Testament 1:17, p. 11] (Udo Schaefer, Infallible Institutions? published in Reason and Revelation: Studies in the Babi and Bahá’í Religions, volume 13, pages 3-37)

From the above extracts it seems fairly clear that letters written on behalf of Shoghi Effendi “related strictly to the Cause and interpretation of the teachings” constitute authoritative Bahá’í text, while any personal advice which they may contain is not binding. The Guardian’s statement that he reviewed every letter written on his behalf without exception makes it clear that the authority of the letters was independent of whatever personal “sufferings” might have been caused by certain secretaries, and that there was no “delegation” whatsoever of his interpretative authority, but merely a use of secretarial assistance for his huge burden of correspondence.
(The Universal House of Justice, 1996 Oct 22, Authentication and Authority)

Like any other decision-making body the Universal House of Justice is dependent on information, but the quality of this information varies according to the level on which the decision is made. At the executive and judicial levels, knowledge of the historical facts of a specific case is required, while at the legislative level what is needed is general knowledge of the matters to be regulated. As to the first category, the ascertainment of historical facts, the House of Justice is dependent on others. The factual information necessary for executive and judicial decisions is gathered by subordinate institutions or individuals. An infallible decision would require that, in every case, the factual information provided were absolutely error-free. How should that be possible? There can be no guarantee that all the facts relevant to the decision are indeed compiled, and that these are correctly assessed as to their respective importance before being conveyed to the Universal House of Justice. If this cannot be guaranteed, then there can be no guarantee on the absolute correctness of decisions made at these levels. The correctness of any decision in such an instance is conditional: it depends on the correctness and absolute reliability of the information provided concerning the matter in hand. Conditional infallibility, however, is a contradiction in terms. A decision that is based on fragmentary or wrongly transmitted relevant facts cannot be guaranteed to be correct. The Universal House of Justice’s statement that a decision can be “corrected” when “new facts emerge” confirms this interpretation.
(Udo Schaefer, Infallible Institutions? published in Reason and Revelation: Studies in the Babi and Bahá’í Religions, volume 13, pages 3-37)

Moreover, such a concept of infallibility also has far-reaching psychological implications affecting the consciousness of the believers. An unreflected, even magical vision of the unerring guidance which has been conferred on the House of Justice currently prevails in the community. Some imagine the community to be in possession of some kind of Delphic Oracle, to which everyone can appeal whenever they are in a quandary. This is an utterly unacceptable attitude that fosters the frequently shown inclination to avoid making one’s own decision and to escape one’s own responsibility by submitting difficult matters to the Universal House of Justice in order to get “infallible guidance.” This attitude and way of thinking is irrational. Furthermore it reveals that one presupposes that the Universal House of Justice does not operate in a rational way and does not decide after having conducted a rational consultation, but rather acts as a mere recipient, transformer and mouthpiece of the Holy Spirit; that its decisions are revelational in character. In reality decisions do not come about through quasi-prophetic inspiration ("quasi per inspirationem", “Divino afflante spiritu"). Instead, they are reached in the course of a rational discursive process in which, subsequent to clarification of the normative guidelines set out in the scripture and the establishment of the relevant facts, a formal process of consultation leads to a consensus of opinion and finally to a decision reached by majority vote or by unanimity. This means that the Holy Spirit does not act as a deus ex machina. Information on every decision must be prepared and every decision must be the subject of consultation. Shoghi Effendi has expressed this in an incontrovertible statement: “They may, indeed they must, acquaint themselves with the conditions prevailing among the community, must weigh dispassionately in their minds the merits of any case presented for their consideration,” then “they are to follow, in a prayerful attitude, the dictates and promptings of their conscience.” [World Order p. 153] Thus, infallibility is not, as the Universal House of Justice has expressly stated synonymous with omniscience, nor does it preclude rational consultation and judgement.
(Udo Schaefer, Infallible Institutions? published in Reason and Revelation: Studies in the Bábí and Bahá’í Religions, volume 13, pages 3-37)

Shoghi Effendi was asked several times during his ministry to define the sphere of his operation and his infallibility. The replies he gave and which were written on his behalf are most illuminating. He explains that he is not an infallible authority on subjects such as economics and science, nor does he go into technical matters since his infallibility is confined to ‘matters which are related strictly to the Cause‘. He further points out that ‘he is not, like the Prophet, omniscient at will‘, that his ‘infallibility covers interpretation of the Revealed Word and its application’, and that he is also ‘infallible in the protection of the Faith‘. Furthermore, in one of the letters, the following guideline is set forth:
"‘... It is not for individual believers to limit the sphere of the Guardian’s authority, or to judge when they have to obey the Guardian and when they are free to reject his judgment. Such an attitude would evidently lead to confusion and to schism. The Guardian being the appointed interpreter of the Teachings, it is his responsibility to state what matters which, affecting the interests of the Faith, demand on the part of the believers, complete and unqualified obedience to his instructions.
(Universal House of Justice, Lights of Guidance, p. 311)

The Universal House of Justice in arriving at a decision needs to have before it all the facts involved in the matter. If, after making a decision, new facts emerge, it may well be changed (Letter on behalf of the Universal House of Justice, dated 13 August 1981).

The infallibility of the Guardian is confined to matters which are related strictly to the Cause and interpretation of the teachings; he is not an infallible authority on other subjects, such as economics, science, etc. When he feels that a certain thing is essential for the protection of the Cause, even if it is something that affects a person personally, he must be obeyed, but when he gives advice, such as that he gave you in a previous letter about your future, it is not binding; you are free to follow it or not as you please.
(Shoghi Effendi, Directives from the Guardian, p. 33-34)

What is the purpose of an infallible legislature? The seal of immunity to error means that the law passed by the Universal House of Justice constitutes sacred law, which is qualitatively different from any ius humanum and is ascribed in the hierarchy of law to the divine law (ius divinum). Infallibility is expressed in different categories. The sphere of interpretation is governed by the categories “true-false.” An infallible interpretation is one that is absolutely “true;” it is a manifestation of divine truth. The law enacted by an infallible legislature is not “true” but rather “just.” Therefore an infallible legislation means an enactment of legal norms that are in accordance with divine justice.
(Udo Schaefer, Infallible Institutions? published in Reason and Revelation: Studies in the Bábí and Bahá’í Religions, volume 13, pages 3-37)

With regard to decisions taken by the Universal House of Justice itself, instructions it issues, and the relationship of these to the information supplied, it is obvious that the nature of a decision or instruction is affected by the information on which it is made.
(Letter on behalf of the Universal House of Justice, dated 26 May 1993).

Your first question stems from a statement made by an individual on an Internet Bahá’í discussion group which asserts that “Shoghi Effendi has allowed for the Universal House of Justice reaching a wrong decision.” In describing the House of Justice, ‘Abdu’l-Bahá states clearly, “Whatever will be its decision, by majority vote, shall be the real truth, inasmuch as that House is under the protection, unerring guidance and care of the one true Lord. He shall guard it from error and will protect it under the wing of His sanctity and infallibility. He who opposes it is cast out and will eventually be of the defeated. It is the exclusive sphere of the Universal House of Justice to “pronounce upon and deliver the final judgment on such laws and ordinances as Bahá’u’lláh has not expressly revealed.” It carries responsibility for the application of the revealed Word, the protection of the Faith, as well as the duty “to insure the continuity of that divinely-appointed authority which flows from the Source of our Faith, to safeguard the unity of its followers and to maintain the integrity and flexibility of its Teachings.” However, the Universal House of Justice is not omniscient, and the friends should understand that there is a difference between infallibility and omniscience. Like the Guardian, the House of Justice wants to be provided with facts when called upon to render a decision, and like him it may well change its decision when new facts emerge, or in light of changed conditions at some point in the future. We have found nothing in the writings of Shoghi Effendi which suggests that the House of Justice would on any occasion reach a “wrong decision”.
(The Universal House of Justice, 1996 Jun 14, Infallibility, Women on House of Justice)