I say, and with God's success: In the case of a mistake, the person who errs is aware of their fasting but makes an error; for example, they may swallow water while rinsing their mouth instead of spitting it out. As for forgetting, the person who forgets is not at all aware of the fast, so they break their fast under the impression that they are not fasting. The one who makes a mistake invalidates their fast and must make it up, unlike the one who forgets, who does not have to make it up and is excused. And God knows best.