Possibility Of Forgery In The Chain Of Transmitters (The Isnad)
Yes, it is quite possible. That is why the whole ilm-ur-rijal came into existence to mitigate this risk. The reason we have rigorous principles in place while approaching any Hadith ( Please see : Discussion 56230 ), is to make sure that these limitations in individual reportings could be minimized. But still, they could never be eliminated completely because despite these controls in place, certain inherent limitations embedded with this particular mode of transmission (i.e. Akhbar-i-ahad ) could never be overcome. That is why we can never attain absolute assurance from Hadith; at best, after applying all the principles of understanding Hadith very carefully, we can attain only reasonable assurance to some extent ( Please see : Discussion 85583 ).