al-Haafiz Ibn Hajar (may Allah have mercy on him) and other scholars are not infallible or free of error. Not one of them is infallible. I wrote some comments on his book al-Fath al-Baari, from the beginning of it to Kitaab al-Hajj, and I noted my reservations about some points (may Allah have mercy on him).
The point is that he is not infallible, and scholars who were greater than him were not infallible either. When the truth becomes apparent, then truth is the lost property of the believer. If proof is established concerning some matter, then we must follow that for which proof has been established from the Book of Allah or the Sunnah of His Messenger (blessings and peace of Allah be upon him), even if it goes against the view of a great imam, an imam greater than al-Haafiz Ibn Hajar, and even if it goes against the view of some of the Sahaabah. Allah says (interpretation of the meaning):
“(And) if you differ in anything amongst yourselves, refer it to Allaah and His Messenger (صلى الله عليه وسلم)…”
He did not say: refer to so-and-so or so-and-so, rather He said “refer it to Allaah and His Messenger (صلى الله عليه وسلم), if you believe in Allaah and in the Last Day. That is better and more suitable for final determination.” And He said (interpretation of the meaning):
“And in whatsoever you differ, the decision thereof is with Allaah (He is the ruling Judge)”
But it is essential to be certain of the issue and show respect the scholars and observe proper etiquette with them. If a person finds a weak view on the part of one of the imams or scholars or respected muhadditheen, then that does not detract from their status, and he must respect the scholars, observe proper etiquette with them, speak good words and not revile them or scorn them. Rather he should explain the truth with evidence, whilst also praying for the scholar, seeking Allah's mercy for him and asking Allah to pardon him.
This is how the attitude of people of knowledge should be towards other people of knowledge: they should appreciate the people of knowledge, and acknowledge their status, position and virtue.
But that should not prevent them from pointing out mistakes, if they find an obvious mistake, whether on the part of a classical scholar or a modern one. The scholars are still debating with one another until today, and will continue to do so until the Day of Resurrection. Imam Maalik (may Allah have mercy on him) said: There is no one among us but he may refute or be refuted, except the occupant of this grave -- meaning the Messenger of Allah (blessings and peace of Allah be upon him).
Imam al-Shaafa’i (may Allah have mercy on him) said: The scholars are agreed that the one to whom the Sunnah of the Messenger of Allah (blessings and peace of Allah be upon him) has become clear has no right to forsake it in favour of the view of any person.
Imam Ahmad (may Allah have mercy on him) said: I am amazed at people who know the isnaad and its soundness -- meaning from the Prophet (blessings and peace of Allah be upon him) -- but they follow the view of Sufyaan -- meaning al-Thawri. Sufyaan (may Allah have mercy on him) was a great imam, yet despite that, Ahmad criticised the one who ignores the hadeeth and follows his opinion. Then Imam Ahmad (may Allah have mercy on him) recited the verse (interpretation of the meaning): “And let those who oppose the Messenger’s (Muhammad صلى الله عليه وسلم) commandment (i.e., his Sunnah – legal ways, orders, acts of worship, statement, etc.) (among the sects) beware lest some Fitnah (disbelief, trials, afflictions, earthquakes, killing, overpowered by a tyrant, etc.) befall them or a painful torment be inflicted on them” [al-Noor 24:63].
Imam Abu Haneefah (may Allah have mercy on him) said: If there comes a hadeeth from the Messenger of Allah (blessings and peace of Allah be upon him), then we accept and follow willingly, and if there comes a report from the Companions (may Allah be pleased with them), then we accept and follow willingly, but if there comes a report from the Taabi’een, then we are men and they are men (i.e., we are on an equal footing with them).
And the scholars said a great deal concerning this matter. And Allah is the source of strength. End quote.