Many of the Mujasima consider singular reports to be decisively reliable (Qat’i Al-Thubut). This claim is undoubtedly mistaken, and Al-Suyuti explains why:
|And if it is said: “this Hadith is Sahih” what is meant: that which is supported by a connected Sanad, with the qualities mentioned earlier. So we accept to act upon it [the Hadith] because of what is apparent from the Isnad. However, this does not mean that this Hadith is decisively reliable in of itself. Since it is possible for a trustworthy narrator to make a mistake or forget.|
-Al-Suyuti, Tadrib Al-Rawi (pg. 75)
Comment: not only is it possible for a trustworthy narrator to make a mistake or forget, but (in most cases) our belief in the trustworthiness of this narrator is itself also probabilistic.