What is the ruling on someone who said to his wife, "You are unlawful to me," while ignorant of the rulings on zihar, and then did the act he swore to, out of forgetfulness? Is that considered zihar or an expiable oath?

If you swore by declaring your wife unlawful, but you did not intend divorce (talaq) or a revocable divorce by comparing her to an unmarriageable relative (zihar), then its ruling is the same as swearing by Allah. No divorce or zihar takes place by acting upon it. If you acted upon what you swore about out of forgetfulness, scholars have differed regarding the necessity of expiation (kaffarah). The preponderant opinion according to us is that it is not obligatory. This is supported by the statements of Al-Nawawi, Ibn Taymiyyah, and Al-Bahuti. However, if you perform expiation as a precaution, it is good.