Fragments of Shaykh Mufid's Life‏

I'm going to quote this directly from the 12r Shia sites.
Here is proof that he had a Mutazilli teacher.

He was born in the year 338 A.H./949 and was brought up in a village. His father brought him to Baghdad for his education. There he studied under Shi'i and Mu'tazili scholars. He showed such promise that one of his teachers recommended that he study under one of the leading scholars of the period, 'Ali b. 'Isa al-Ramani. He also studied under the leading Shi'i traditionists of the time, al-Shaikh al-Saduq.

http://www.al-islam.org/al-serat/irshad-howard.htm

Note: Ali bin Isa Al-Ramani is Mutazilli scholar

Here is another shocking discovery, and it tell us why Shaykh Mufid went to the Mutazilla to learn ilm.

The Sheik went to two erudite scholars, Ibne Abd Allah and Ibne Abi Jafar, who were impressed with his knowledge that they sometimes felt that they themselves were unable to reply to the queries of the young student. It is narrated that once Ibne Jafar expressed his inability to reply to his queries and asked Sheik Mufeed to rather meet Ali ibne Eesa and seek his guidance. Ali ibne Eesa Rumani was the authority on Ilm e Kalam in Baghdad those days.
http://www.islam-laws.com/marja/mufeed.htm

Fiqhi 12r Debate Between Shaykh Mufid and Syed Murtaza. Its laughable....

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).
http://www.islam-laws.com/marja/mufeed.htm

Note: If the 12th imam appeared that would have counted as the second witness.

Recall earlier an Imaginary Bibi Fatima (sa) appears and calls Shaykh Mufid her own Shaykh.

Ibn Abil Hadeed al-Mo'tazaly in his commentary on Nahjul Balaghah writes that once Sheikh Mufid saw Fatima al-Zahra, peace be upon her, in his dream. She was accompanied by her two young sons, al-Hasan and al-Husain, peace be upon them. Addressing him, she said: "O my Sheikh, teach Fiqh (Jurisprudence) to these two boys of mine." Next day, Fatimah, the mother of Seyyid Murtadha and Syed al-Radhi came to Sheikh, holding hands of her two young sons, and uttered the same words which Fatemah al-Zahra, peace be upon her, had uttered in his dream.


http://www.al-islam.org/amali/2.htm


Notice, how the pronoun 'my' is used before Shaykh.

The 12th Imam Saves Shaykh Mufid from an Invalid Fatwa.
The 12r character assassinate Umar (ra) for depending of Imam Ali (as) too much, but here they claim the 12th imam saved Shaykh Al Mufid from an invalid fatwa.
The greatness of the Sheik may well be gauged from this incident. In the same mosque at Baghdad, where he used to conduct his classes, he was seated when a villager arrived and posed a question to him. He said that a woman had, just then, died in the village and she had a live fetus in her stomach. There was a dispute amongst the villagers as to whether they should bury the fetus along with the dead mother or extricate the baby live and then intern the woman. Some people say that saving the life of the baby was important and others say that conducting any such operation would be disrespect to the dead person. The Sheik, of course, was a fallible being, and at that moment somehow it came to his mind, he told to the villager to go and bury the dead woman in the same condition that she was. The man immediately set forth for the village. When he reached near the village he heard someone calling him from a distance. He stopped. One person, riding on a horse, came near him and said that the Sheikh’s Fatwa was changed and he had asked to deliver the fetus live from the mother’s womb and then the dead body may be buried. He acted on the fresh edict. After some days the villager again visited Baghdad and called on the Sheik. He told him that they had acted according to the instruction conveyed through the rider and had delivered the baby live before interning the mother. Sheik Mufeed understood who it was who corrected his error! The Sheik told to himself, “O Sheik! Your mind has stopped functioning that you should stop giving Fatwa. Your error would have been the cause of the death of a Muslim” From that day he stopped giving Fatwa. After some time he received a letter from the Imam (a.s) asking him to continue giving Fatwa. He wrote that it was the Sheik’s duty to give Fatwa and if he made any errors, the Imam (a.s) would correct them. Such a great personality was the Sheik that he was guided from time to time by Janab e Syeda (a.s) and the Imams (a.s).
http://www.islam-laws.com/marja/mufeed.htm