What is the ruling on one who swore truthfully that they saw something, then it became clear to them that what they saw was mistaken?

Your oath concerning the aforementioned matter is not a yamin ghamus (a false oath sworn knowingly) because you believed yourself to be truthful. Rather, it falls under the category of laghw al-yamin (a vain or unintentional oath), for which Allah does not hold one accountable, as Allah Almighty says: "Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what your hearts have earned." There is no expiation for an oath sworn concerning a past event if the swearer believed it to be true, but it later turned out to be otherwise, for it is considered laghw al-yamin.