What is the ruling on swearing falsely about a forgotten matter, and is expiation obligatory along with repentance?

If an oath was uttered unintentionally, it falls under the category of a 'vain oath' (laghw al-yameen), for which no expiation is required according to many scholars, based on the Almighty's saying: "Allah will not impose blame upon you for what is unintentional in your oaths." Furthermore, doubt concerning the occurrence of an oath or its breach entails no obligation, as the default principle is the absolution of one's responsibility.