Do you not see those who claim to be purified? No, Allah purifies whoever He wills. They will not be wronged by so much as the smallest speck. (Surat an-Nisa', 4:49)
Infallibility and perfection are qualities unique to Allah. We may forget, become heedless, and make mistakes, but Allah does not. Such traits indicate our weakness and need of Allah in every situation. Given this, believers feel regret once they recognize their mistakes, repent, and are careful not to repeat them. Just considering themselves sinless and faultless is a great sin in itself. One verse reads:
To whoever avoids the major sins and indecencies—except for minor lapses, truly your Lord is vast in forgiveness. He has the most knowledge of you when He first produced you from dust and when you were embryos in your mothers' wombs. So do not claim purity for yourselves. He knows best those who guard against evil. (Surat an-Najm, 53:32)
Sincere believers are conscious of their mistakes and weaknesses. For this reason, they repent continuously and pray for Allah's mercy and good pleasure. In return, Allah covers their mistakes, forgives their sins, purifies them, and enables them to attain a superior position.