This refers to the Israelite's breach of covenants and promises, and the bad consequences of their wrongs.

      Because of their break of promises, God cursed them and withdrew His overflowing Mercy from them, as a result of which their hearts were hardened and turned to pieces of stones! So much so that they began to sin against God and His Messengers; misusing their Scriptures. Taking words out of their right postions and meanings, or applying them to the things which the words did not mean that. They misrepresented, and misexplained the words of God to common people and laymen. During that, they forgot even the purpose of God of the revelations.

      This forgetting matters of the srciptures may refer to the fact that in the course of time and history, the Scriptures were lost, and the learned Jews began to revive and rewrite it, and in the course of their action, they forgot a part of the Book and perverted other parts. That which was written and prepared later, was in fact a part of the real Bible of Moses, mixed and mingled with a lot of superstitions.

      Finally the verse (No. 13) orders our prophet Mohammad S.A. to excuse them and to overlook and forget their harms that they did to him in the lifetime of their hostilities, which ended to their failure and shame.