In the Name of Allah, The Most Gracious, Most Merciful
Since Hajj is ordained by Allah on those who are capable once in a lifetime, if a pilgrim violates its rites and fails to perform one of its requirements or if he neglects a sunnah, if he sticks to a sunnah that has led to the violation of something prohibited, or if he commits an act of disobedience, if he performs a prohibited action, his pilgrimage would be null and void or he may have to sacrifice an animal, or he if he does wrong, if he neglects to do something, if he fails to perform what is a priority - if he does that he would waste a unique opportunity that is not repeated, an opportunity to have his last sins forgiven and to qualify him to be admitted to Allah's paradise. All this is due to ignorance, which is the utmost enemy of man. An ignorant person may harm himself beyond what his enemy may harm him. Therefore we say: dear pilgrims learn the provisions of pilgrimage before you go on Hajj. Thus one knowledgeable person is more difficult for Satan to seduce than a thousand worshippers, and a little knowledge of the principles of the deen of islam is better than a lot of worship:
"So ask of those who know the Scripture [learned men of the Taurat (Torah) and the Injeel (Gospel)],if you know not"
And just as awaiting the time of Salah is considered a part of Salah, similarly preparing oneself for Hajj (learning the Hajj rituals before going to Hajj) is a part of the Hajj. Many pilgrims have failed to acquire the necessary knowledge of Hajj before going on pilgrimage and they returned from Hajj without having performed the basic tawaf (circumambulating the Ka'bah) with the result that their pilgrimage would be null and void. Many a pilgrim has crossed the miqat (local point of ihram; official commencement of the duties of Hajj) without having put on the Ihram garments with the result that he needed to sacrifice an animal. And how many a pilgrim has violated the rules of Ihram thinking that he was on the right track...