Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
If a pilgrim in a state of ihram wears an `abaya after stoning Jamarat al-`Aqaba out of forgetfulness or ignorance, he is not blameworthy; his hajj is valid and he is not obliged to slaughter in compensation.
Answering your question, Dar Al-Ifta Al-Misriyyah, states:
It is established in the Shafi`i school of jurisprudence that it is not obligatory for a pilgrim to make a fidyah (compensation) for committing out of forgetfulness or ignorance any of the prohibitions of ihram that are prohibited for reasons of self-indulgence.
These include using perfume, engaging in sexual intercourse, wearing sewn garments and covering the face or head. The fidyah is only obligatory upon a pilgrim who knowingly and deliberately does any of the above.
Based on this, if a pilgrim in a state of ihram wears an `abaya after stoning Jamarat al-`Aqabah out of forgetfulness or ignorance, he is not blameworthy; his hajj is valid and he is not obliged to slaughter in compensation.
Almighty Allah knows best.
Source: http://www.dar-alifta.org/Foreign/ViewFatwa.aspx?ID=1356