In the Mahabharata, it's mentioned that Arjuna forgot the Bhagavad Gita which was once heard in the war field from Lord Krishna. Bhagavad Gita is one of the greatest messages or advices from Lord Krishna to the whole world and it was advised to Arjuna to do his Karma.
The son of Prithâ, after becoming possessed of his kingdom (in an) undisturbed (state), enjoyed himself in the company of Krishna, full of delight in that heavenly palace. And once, O king! they happened to go, surrounded by their people, and rejoicing, to a certain portion of the palace which resembled heaven. Then Arjuna, the son of Pându, having surveyed with delight that lovely palace, in the company of Krishna, spoke these words: 'O you of mighty arms! O you whose mother is Devakî ! when the battle was about to commence, I became aware of your greatness, and that divine form of yours. But that, O Kesava! which through affection (for me) you explained before, has all disappeared, O tiger-like man! from my degenerate mind. Again and again, however, I feel a curiosity about those topics. But (now), O Mâdhava! you will be going at no distant date to Dvârakâ.
But after the Mahabharata war, one day Arjuna told that he had forgotten the Bhagavad Gita. Why did this happen? How can a man forget the greatest advice which led to the success of his life? Was there a mistake in the way Arjuna grasped the Gita that led to this incident?