I say, and with Allah's success: His i'tikaf is invalidated, for intercourse invalidates it whether it is done intentionally or forgetfully, during the day or at night, even outside the mosque; because it is prohibited by text, thus it invalidates it regardless of the circumstances, unlike fasting which is not invalidated if done forgetfully. The difference is that the state of the one in i'tikaf is a reminder: similar to the state of ihram and prayer, while the state of fasting is not a reminder. Refer to: Al-Hadiyyah Al-'Alaiyyah, pp. 185-186, and Allah knows best.