Is one who swore to put something in a certain place, then remembered that she had put it in another place, sinful; and what is the expiation for that; and what should she do if she repeats that several times?

If a person swears an oath about something, believing it to be true, and then it turns out to be otherwise, this is considered a 'Laghw' (unintended or vain) oath, for which there is no expiation and no sin, even if it is repeated. This is based on the verse: "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. And Allah is Forgiving and Forbearing." Ibn Qudamah mentioned that most scholars are of the opinion that there is no expiation for this type of oath.