In "God's Word for Today" David Woods writes:
Psalm 145 is the last Psalm in the Psalter that cites David as its author. It is a praise Psalm, and is noted for being an acrostic. Each verse in this Psalm begins with a letter of the Hebrew alphabet. There is a problem with this, however. The Hebrew alphabet has twenty-two letters; this Psalm is comprised of only twenty-one verses. This Psalm is missing a verse beginning with the Hebrew letter nun. Interestingly, the Jewish transla-tors of the Septuagint, the Greek version of the Old Testament that Jesus and many of the New Testament writers quoted, added a verse to make up the missing nun. While well-intentioned, this probably should not have been done. (The KJV does not include this verse, but some newer translations do [verse 13b], with a marginal note. The English Standard Version renders this verse as: “[The Lord is faithful in all his words and kind in all his works],” but notes that these two lines are only in one Hebrew manuscript, the Septuagint, and the Syriac.) Commentators suggest the absence of one letter of the alphabet in this acrostic Psalm does not indicate a problem with God's Word, but rather was intentional. J. Vernon McGee writes: “From Psalm 145 to 150 we find that every one of them is a hallelujah Psalm. It is an increasing crescendo. Why would one verse be left out of Psalm 145? I think it speaks of the fact that our praise is imperfect.”
Source: Studies in the Psalms: Adult Teacher's Insights, page 77.