There is a very interesting incident of a disciple giving an edict (Fatwa) contrary to that of his mentor!
It is said that on the saddle of the steed of Sayed Murtada a dog had passed urine. Sheik Mufeed, who had witnessed the scene, informed Sayed Murtada that the saddle was rendered impure (Najis) with the dog’s urine.
Sayed Murtada replied that whatever Sheik Murtada said must be true, but his own Fatwa (Verdict) was that there must be at least two witnesses giving evidence to agree about the event happening. Since the Sheik was the only witness, his evidence wasn’t sufficient.
The argument prolonged and in the end it was decided to seek a solution to the problem from Hazrat Amir al Momineen (a.s). Hence both the persons proceeded to Najaf e Ashraf.
There are three different narrations regarding this event. They wrote their problem on a piece of paper and left it in the Zarih (Sarcophagus) of Amir al Momineen (a.s).
One narration is that when the mentor and the disciple reached the mausoleum next morning, they heard the voice of Hazrat Amir al Momineen (a.s) saying “Ya Sheik anta motamadi wal haq maa waladi ( O Sheik! You are the secretary, but the truth is with the son!).
The second narration was that the paper was recovered on which was written the same verdict.
The third narration was that both, the mentor and the disciple, dreamed in their sleep when Hazrat Ameer al Momineen (a.s) came and said that the truth was with his son and also that he (a.s) had total faith in his secretary (Sheik Mufeed).
Shi‘ite Authorities in the Age of Major Occultation Part 1: Sheikh Mufid