What is the ruling on one who breaks an oath out of forgetfulness, and is expiation for the oath obligatory upon him?

Scholars have differed concerning one who does the thing he swore not to do, forgetting his oath: Some have gone to the opinion that he does not break his oath and owes no expiation, due to the pardon for error and forgetfulness. Others have gone to the opinion that he does break his oath and owes expiation, because his intentional doing of the act necessitates expiation. The more cautious approach is to offer expiation, in order to avoid the disagreement.