How can a believer's deeds be nullified without him realizing it, even though Allah Almighty does not hold us accountable for what we err in?

The verses warn against bad actions that may nullify deeds, such as committing forbidden acts while knowing they are forbidden, or showing off and seeking reputation, even if the doer does not perceive the nullification of their deeds. So, the meaning of His saying, the Exalted: "while you perceive not" is that you do not perceive the nullification, not the action itself. For punishment, his knowledge of the act's prohibition suffices, even if he does not know that it nullifies deeds. The verse does not indicate that the deeds of one who is excused or mistaken are nullified. Furthermore, the warning therein pertains to future actions, not the nullification of past deeds committed before awareness of their prohibition. The nullification of deeds may be due to disbelief or sins, and what is often meant by it is a decrease in faith and the nullification of some acts of worship, not disbelief itself, for disbelief only occurs with intention, choice, and knowledge. The importance of the matter lies in preserving deeds from what corrupts them and nullifies them.