What is the ruling on someone who swore an oath of divorce upon his wife that she should not do something, then forgot his oath and allowed her to do it? Does the divorce take effect? And what is the ruling if the wife also forgot and did the sworn-upon thing?

An oath by divorce carries the ruling of a conditional divorce. So, if a husband makes his wife's divorce conditional upon an action, and she performs it, the divorce occurs according to the majority of jurists. Ibn Taymiyyah, however, holds that an expiation for an oath is due if the husband intended a threat and not an actual divorce. The husband's forgetfulness of the condition has no bearing. But if the wife performs the action forgetfully, and she is someone who takes the condition seriously, then some jurists hold that the divorce does not occur.