The proper formation of one's conscience is a lifelong process and responsibility. Even one who seeks to make correct judgments in conformity with God's Will is at risk of making the wrong choices; erroneous judgment. Whether we acknowledge it or not, we are influenced throughout our lives by our family, friends, the media, and the environment we grow up in among many other factors. So even if we make a sincere effort to understand how God speaks to us and to prepare ourselves using the "tools" available to us as Christians, because of these influences we are still susceptible to erroneous judgement.
The propensity for erroneous judgement is increased by presuppositions of false assertions, an insincere quest for or even absence of the pursuit of a properly formed conscience, bad examples, a lack of true charity, and ignorance of and especially rejection of Christ, the Gospel, and the authority of the Church.
It is true that we are creatures of habit. The more we become accustomed to making wrong choices, the more comfortable we will be making them. As I discussed in "Are you convicted, accused, or left alone?", we can even lose any sense of our "moral compass". However, the more we choose correctly, the stronger bearing we have on magnetic North, or in this case, "magnetic morality".
Hence the more right conscience holds sway, the more persons and groups turn aside from blind choice and strive to be guided by the objective norms of morality. (Gaudium Et Spes)
The conclusion of this series will appear Monday covering a few "rules".