Is expiation (kaffarah) obligatory upon one who swore a false oath based on a presumption, then it became clear to him that his oath was true, or upon one who swore an oath unconsciously then remembered that it was incorrect?

A Muslim should not swear excessively, and should only swear by Allah the Almighty.

There is no expiation for an oath taken by a person believing it to be true, only for it to be later revealed as false; because it falls under the category of a "vain oath" (laghw al-yamin) according to most scholars. Similarly, oaths made unintentionally or unconsciously are also considered vain oaths and require no expiation.

However, if you swear to do something or to refrain from doing something in the future, and then you change your mind and break your oath, expiation becomes obligatory.