The Obligation of sacrificing an Animal is not lifted from whoever leaves off an Obligation
Question :
Is the obligation of sacrificing an animal lifted from the ignorant person who does not know a ruling or the person who left off one of the obligations of Hajj due to forgetfulness, like spending the night (at Mina), stoning, and shaving the head, or is the sacrifice mandatory? Likewise, what about the situation of the person who committed one of the prohibitions of Ihram?
Answer:
The obligation of sacrificing an animal is lifted from the ignorant person or the person who forgot and committed one of the prohibitions of Ihram. However, the obligations is not lifted from whoever left off one of the obligations of Hajj or 'Umrah due to ignorance or forgetfulness. This is due to the statement of Ibn 'Abbas: "Whoever leaves off a Hajj rite or forgets it, then he must slaughter an animal." This is also due to the Hadith of the man who wore a cloak that was perfumed with fragrance while he was performing 'Umrah.
Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 4 Pages 260-261