Someone, a sheikh, said that it is not incumbent to follow only one Imaam because even the great Imaams have made errors.

Answer

His reasoning is deceptive. It is incumbent to follow one Math-hab 100%. It is not incumbent to follow 100% one Imaam of a Math -hab. The Math-hab has to be followed. Whilst it is accepted that all humans, even great Fuqaha and Imaams err, the official ruling of the Math-hab has to be followed. For example, if Imaam Hambal had erred (as this someone mentioned) on the issue of the oath, then the Fuqaha of the Hambali Math-hab would have set it aside. They would never issue a ruling on the basis of an error.

If Imaam Abu Hanifah for example had erred in a ruling, then there were thousands of Hanafi Fuqaha in all times to set the error aside. It is impossible to imagine that thousands of Fuqaha of the highest standing will enact Ijma' (Consensus) on an error when they are aware of the Haqq.

A layman cannot say that a great Imaam has erred in his ruling. On what basis does he say that Imaam Hambal, for example, had erred? What the sheikh said is not binding. To understand if Imaam Hambal had erred, one has to look at the official position of the Hambali Mat-hab. If the Fatwa of the Hambali Math-hab is in conflict with what Imaam Hambal had said, then it will be understood that he had erred, hence the Hambali Fuqaha had unanimously set aside that particular view or error.

The official position of the Math-hab will always be correct. We do not go according to every view of Imaam Abu Hanifah, for example. And so is it with all the other Math-habs. The ruling of the Jamhoor Fuqaha of the Math-hab is the official stance for practical implementation. This stance has been commanded by the Imaams to their Students. Thus, even if the error of the Imaam is set aside, we shall still be following his Math-hab 100%.