Adam Clarke on Psalms 40:12

Innumerable evils have compassed me about - This part does not comport with the preceding; and either argues a former experience, or must be considered a part of another Psalm, written at a different time, and on another occasion, and, were we to prefix the two first verses of the seventieth Psalm to it we should find it to be a Psalm as complete in itself as that is.

They are more than the hairs of mine head - This could not be said by any person who was exulting in the pardoning mercy of God, as David was at the time he penned the commencement of this Psalm.