What invalidates the i'tikaf besides prohibited acts?
General Chapter
Al-Mughni
Book of I'tikaf
Primary text
If a person apostatizes, the i'tikaf is invalidated due to the verse {If you associate others with Allah, then surely your work will be nullified} (Quran 39:65). Similarly, if one consumes intoxicants, the i'tikaf is invalidated because one is then excluded from being a resident of the mosque.
Supporting text
If i'tikaf is voluntary, no makeup is required upon invalidation, as voluntary acts are not binding upon commencement except for Hajj and 'Umrah. If it was vowed, if it was a vow for consecutive days, the preceding valid portion is nullified, and it must be restarted. If it was a vow for specific days, there are two views: either the past portion is voided, or it stands as performed correctly, requiring only makeup and expiation for the breach.