How can one be certain of the authenticity of an Ahad (solitary) Hadith, considering the possibility of error or forgetfulness on the part of the narrators, especially if the Hadith is Gharib (uncommon/strange)?

It is not permissible to doubt the majority of the Sunnah (definition) on the pretext of the possibility of a narrator's error, because the majority of the Sunnah consists of isolated (Ahad) narrations, which would mean building the religion upon doubt and illusion. Scholars have unanimously agreed on accepting and acting upon solitary narrations, and they have condemned those who stipulated multiple narrators, with the exception of the people of innovation. Allah has guaranteed the preservation of the Sunnah, and Hadith (definition) scholars have meticulously tracked errors and falsehoods within it.