What is the ruling on one who swears an oath about something – without making a vow – and the opposite of what he swore happens?

Scholars have differed regarding one who swears an oath concerning a future matter, believing it to be true, but it turns out to be otherwise. Most of them hold that expiation (kaffarah) is obligatory, while Sheikh al-Islam Ibn Taymiyyah chose that it is not obligatory, considering it a 'laghw' (unintentional or vain) oath. The more cautious approach is to pay the expiation to clear one's conscience and to avoid disagreement. One should be wary of excessive swearing and occupy oneself with what benefits one in this world and the Hereafter.