It is not a condition for an ally of Allah that he be free of (protected from) mistakes and errors. It is quite possible that some knowledge of the shari'ah may be hidden from him, just as it is possible for him to be confused about some issues in Islam But, he may not necessarily because of this, leave the ranks of allies of Allah, since Allah has overlooked and forgiven for this Ummah error, acts of forgetting, and things done under compulsion:
"There is no burden upon you for that in which you were mistaken, rather that which you have done with the full determination of the heart." [Holy Quran Soorah Ahzab (33): 5]
Abu Hurairah (may Allah be pleased with him) said: "Prophet (sallalahu alaihe wa-sallam) said:
"When a ruler exerts himself to arrive at the correct ruling, and is correct, he gets two rewards and when he exerts himself but is mistaken, he gets one reward." [Bukharee and Muslim]
Since, it is possible for any ally of Allah to make a mistake. It can never be obligatory upon the people to believe in everything any one of them says, except that in the case that one of them is a prophet. Rather, it is obligatory for him to measure all of that up to the criterion of that which the Prophet (sallalahu alaihe wa-sallam) brought. If it is in agreement therewith then he can accept it, but if it is in disagreement then he must reject it, and if he cannot be sure whether it is in accordance with the Prophet's message or at variance with it, he must refrain from believing it or applying it