Psalms 18:50

This verse is by many treated as a late liturgical addition to the hymn. The change to the third person is certainly somewhat suggestive of this, but by no means conclusive.
The question of the relation of the two copies of this hymn to each other is far too complicated and difficult for discussion here. Each has been again and again claimed as the original. The best explanation of the variations is that the compositions were independent copies of some original, and that the psalm, like many others, was altered in preparation for the choir use.