Is it possible for a human to err or be forgetful by divine decree, without sin, just as a diligent student might miss a question on an exam because a certain total score is predetermined for him? Or is every error or act of forgetfulness attributable to the human and his devil? And if the first premise is possible, what should we do regarding our mistakes, and how do we distinguish between the two cases?

Everything that befalls a servant, whether good or evil, is by Allah's divine decree (Qada') and destiny (Qadar), as stated in His Almighty saying: "Indeed, We created all things with a [divine] measure." [Quran 54:49] Allah recorded the destinies of creations fifty thousand years before He created the heavens and the earth. The remedy for this is supplication (Du'a), for it repels divine decree and receives affliction. The one who supplicates will not be deprived of good, for Allah will either answer him promptly, or save the supplication for him in the Hereafter, or avert from him an equivalent amount of evil.