Even as we are encouraged to memorize the Qur'an we are encouraged to retain it in our memory by revising it. The Prophet (peace be upon him) said, "Continue to revise the Qur'an you have memorized; for it is more slipping than camels that are tethered."

Therefore, to forget the Qur'an because of one's negligence is a grave sin.  The Prophet (peace be upon him) has warned us against it. The scholars also say that one of the reasons one may forget the Qur'an he or she has memorized could be the sins they are indulging. The scholars say, calamities, that strike us,  are often due to our own sins; and forgetting the Quran could be a penalty for the sins we are indulging in; hence the need to shun all sins, both outward and inward, major and minor.

Having said this, I should also point out: the warning does not apply to those who forget it because of one's preoccupation with his important duties such as seeking knowledge, etc. Ibn Rushd of the Malikite school has cited the consensus of scholars on this. In other words, the warning is not extended to those who forget it because of no deliberate negligence on their part. Allah says, "God does not burden any human being with more than he is well able to bear: in his favor shall be whatever good he does, and against him whatever evil, he does. O our Sustainer! Take us not to task if we forget or unwittingly do wrong! "O our Sustainer! Lay not upon us a burden such as Thou didst lay upon those who lived before us!" (Qur'an: 2:286).  

I pray to Allah to forgive us our trespasses and slip-ups.