ASKING THE IMPORTANT QUESTIONS: Why Is There No Nun Verse in Ashrei? (Mitchell First, Jewish Link of New Jersey). Interesting analysis of the problem of a missing verse (that should begin with the letter nun) in the acrostic poem in Psalm 145.
There's more here on the longer passage about Nahash the Ammonite found in a copy of 1 Samuel 10/11 in one of the Dead Sea Scrolls.