Sa'eed Bin Mansur can be found in Sahih Muslim 1350c, 2382b, 1252a, etc. He is also found in narrations that are present in other collections like Sunan Abu Dawud, Adab Al-Mufrad, Musnad Ahmad, etc. Since all the narrators of Sahih Muslim are Thiqa (Trustworthy), this suggests that Sa'eed Bin Mansur is also a trustworthy narrator. But why did Ad-Dhahabi declared him as weak in Mizan Al-Itidal? Here is an excerpt from Mizan Al-Itidal (See highlighted text in red):-
This reads:- وأما يعقوب الفسوى فقال : كان إذا رأى في خطابه خطأ لم يرجع عنه
Which means:- As for Yaqub Al-Faswa, he has stated, "If he saw a mistake in his (Sa'eed Bin Mansur's) speech, he did not rectify it."
So if Sa'eed Bin Mansur made mistakes, then why is he considred thiqa (trustworthy) by Imam Muslim and why did Imam Muslim included his narrations in his Sahih collection?