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. 
Hence, they were forced to try to change the wording of the Qur'aan. They attempted to change the verse that states,
to "And Moses spoke to Allaah directly (wa kallamAllaaha Moosaa takleemaa)." 
They also quote the Qur'aan out of context or ignore other related verses that give a complete picture of the Quranic view of the topic at hand. When this is done, the Qur'aan can be used to "prove" things that are incorrect.
Here are some examples:
(1) If someone wants to prove that it is obligatory to obey and be loyal to the government, no matter how much the government may be non-Islamic, he may only quote the verse,
and ignore all of the other relevant verses - even the remainder of the this particular verse itself, which reads:
- and ahaadeeth on the same question. In a relevant hadeeth, the Messenger of Allaah صلى الله عليه وسلم stated, "Listening and obeying is a duty as long as one is not ordered to do an act of disobedience [to Allaah سبحانه و تعالى]. If one is ordered to do an act of disobedience, there is then not to be any listening or obeying." 
The Messenger of Allaah صلى الله عليه وسلم also said, "There is no obedience to any human being if it involves sin; obedience is only in that which is right and proper." 
And he صلى الله عليه وسلم said: “There is no obedience to any human being if it involves disobedience of Allaah.” 
(2) Similarly, if someone wants to prove that it is permissible to take Christians as awliyaa' (close friends and allies) , he will only quote the following verse, ignoring the numerous other verses that are pertinent,
Such a person will ignore verses like the following,
(Soorah Aali 'Imraan, 3:28)
(Sooratul-Maa'idah, 5:51) 
This problem of approaching the Qur'aan with one's preconceived notions of right and wrong may even be growing worse in this age of modern science and advancements in knowledge. Some Muslims nowadays spend years of specialised study to get, for example, an advance degree. They master a particular topic. Unfortunately, sometimes this leads them to believe that they completely understand the reality and essence of the field they studied and to start analysing the Qur'aan in their light of "confirmed knowledge." 
It is easy for such a person to forget Allaah's سبحانه و تعالى words,
and to approach the Qur'aan in an improper fashion. Those verses that go against what the person thinks he knows must either be ignored, given a new reinterpretation based on their knowledge or, and this is the worst case, rejected as false in light of today's modern science. 
If a person approaches the Qur'aan in this manner, he will never see its real guidance. He will continue in his obstinate behaviour and wandering ways no matter how often he reads the Qur'aan - unless Allaah سبحانه و تعالى has mercy upon him and guides him to the proper way of approaching the Qur'aan. Until he changes his approach to the Qur'aan, his heart, mind and soul will be closed to its true guidance.
One has to turn to the Qur'aan realising that it is the guidance from Allaah سبحانه و تعالى, the All-Wise, the All-Knowing and it is the Furqaan (criterion of right and wrong) showing the person the difference between right and wrong. 
Again, this was the way of the unique generation of the Companions رضي الله عنهم of the Prophet صلى الله عليه وسلم.
Salaahud-Deen Al-Khaalidi writes, "This is what the noble Companions رضي الله عنهم did in their behaviour towards the Qur'aan. They were a unique Quranic generation. Everyone of them would leave all of his old thoughts, perceptions and traditions on the doorstep; then he would enter the broad and pure world of the Qur'aan with nothing in his hands [that is, no preconceptions] and then he would build himself in a balanced manner, stage by stage and would derive his decisions, culture and way of life from the Qur'aan, and he would leave its study as a person full of faith, balanced and straight." 
To put it into Western terms and to relate it directly to those who convert to Islaam: Many times people enter into Islaam with a lot of "old baggage." It is a must that a person make a complete break from the teachings of jaahiliyyah (ignorance) and discard all of that "old baggage." Otherwise, it may colour his understanding and reading of the Qur'aan. He may be reading the Qur'aan or ahaadeeth and interpreting them in the light of that "old baggage" and, therefore, he never gets a chance to see the whole and complete picture that is being presented to him by Islaam.
In many cases it is not obvious to the reader that he is not approaching the Qur'aan in this correct manner. This is probably the saddest case. This may be particularly true for those who have grown up in contemporary Muslim societies that have had teachings alien to Islaam penetrate their society and culture. A person from those societies may read the Qur'aan in the light of what he believes are Islamic teachings while, in fact, they are not teachings of Islaam. In other words, his vision has been blurred by customs he practices and sees as Islamic but are not, by teachings he has received that he thinks are Islamic but are not, and so on. Every time he comes across a verse related to these topics, he interprets it according to his incorrect notions. Unfortunately, he has never allowed himself a true study of the Qur'aan that will show him the proper Islamic views of these same ideas that he thinks are correct. This person's intention may certainly be good but, unless Allaah سبحانه و تعالى guides him to see the mistake of his notions, he will actually miss the true guidance of the Qur'aan.
In sum, if one really wants to learn from the Qur'aan, he must approach the Qur'aan in such a way that he allows the Qur'aan to speak for itself and tell him what is correct and what is false. All preconceived notions must be judged by what the Qur'aan says and not vice-versa. If the Qur'aan is not approached in this manner, one should not expect to benefit from its teachings related to those matters concerning which the reader has already made his conclusions.