Surah 70. Al-Ma'arij, Ayah 28

This warning against pharisaic self-righteousness implies that however "good" a person may be, there is always a possibility of his or her having done a moral wrong (e.g., an injury to a fellow-being) and then conveniently "forgotten" this sin. Elliptically, this warning contains a call to increasing consciousness in all one's doings - for, "temptation to evil (fitnah) does not befall only those who are bent on denying the truth" 8:25 , but may also befall people who are otherwise righteous.

I.e., that the punishment of sin may come suddenly at any time, when you least expect it.