There is no question that the Qur'aan guides to the straight path and to the truth. This is one of its major goals. Yet, at the same time, although this guidance is perfect and preserved, it must be allowed to guide the reader. The Qur'aan must be given the opportunity to tell its reader what the straight path is. That is, the reader must approach the Qur'aan with, if one may use the expression, an "open mind." If the reader has already decided what is right and what is wrong, and is only looking to the Qur'aan to substantiate his views, then the Qur'aan will never give him the guidance it contains. Indeed, in that case, the Qur'aan would never be given that opportunity.
Unfortunately, it is the case with some readers of the Qur'aan who, instead of letting the Qur'aan speak for itself, have taken it upon themselves to speak for the Qur'aan. If it contains any passage that contradicts their preconceived notions of what is the truth, instead of accepting what the Qur'aan says, they simply try to reinterpret the Qur'aan in such a way that it is consistent with their beliefs and ideas. They do not realise that it is their thinking and beliefs that need to be changed or corrected according to the Qu'raan and not vice-versa. Allaah سبحانه و تعالى, Who revealed the Qur'aan, is not in need of their far-fetched interpretations. It (along with the Sunnah of the Prophet Muhammad صلى الله عليه وسلم of course) presents the truth in clear and plain language and it is simply up to the reader to accept what Allaah سبحانه و تعالى has said in the Qur'aan.
In the history of Islaam, one can see that the lack of allowing the Qur'aan to state what is true and what is false has been a major characteristic of the heretical and stray groups (ahlul-bida'). 
They approached the Qur'aan with preconceived notions and tried to force their opinions on the Qur'aan. 
Without a doubt, it can be stated that they did not turn to the Qur'aan for its guidance - since they had already decided what the truth is. They turned to it only trying to substantiate their views. A glaring example of this nature is the Mu'tazilah  who even attempted to change the wording of the Qur'aan because it was inconsistent with their preconceived beliefs. The Mu'tazilah, due to the supposedly "ingenious" theories they developed, did not believe in Allaah's سبحانه و تعالى attribute of speech. They have a length theory and proof that such an attribute is impossible for Allaah سبحانه و تعالى according according to their notion of Allaah.