Is it obligatory to denounce someone who errs or forgets?

The one who errs and the one who forgets are excused, and the pen of obligation is lifted from them, due to Allah's saying: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended," and His saying: "Our Lord, do not impose blame upon us if we forget or make a mistake." However, advising them may be obligatory in many cases, such as alerting the Imam if he stands for a fifth rak'ah in prayer, or alerting someone who is eating and drinking forgetfully during the day in Ramadan, or someone who is wearing something impure without knowing it, due to the hadith: "Religion is sincere advice." But if the sin was committed in the past, then advice is not obligatory unless the sin resulted in a right owed to another human being.