Does the husband's statement "علي الحرام" (Upon me is the forbidden/unlawful) count as a vain/unintentional oath, or does it necessitate expiation every time?

As long as the husband does not intend divorce or ẓihār (a pre-Islamic form of divorce by likening one's wife to an unmarriageable kin) by his oath, its ruling is that of an oath by Allah. If he frequently swears by what is forbidden, making an oath to do or not to do something, then an expiation is incumbent upon him for each oath if he breaks it. However, if it flows from his tongue unintentionally, it is considered laghw (a vain or unintentional oath). The permissible oath is swearing by Allah the Almighty, as the Prophet, peace and blessings be upon him, said: "Whoever takes an oath, let him swear by Allah, or remain silent."