Ashrei, a bit of liturgy which we read three times per day, is a combination of Psalm 84:5, Psalm 144:15, and Psalm 145. The first part says how fortunate we people are to dwell in the house of the Almighty, followed immediately by a restatement saying how blessed we are. The Psalm 145 section, then, is the familiar acrostic portion, in which each line begins with the next letter of the alef-bet.
Except, as the school children will tell you, there is no “nun.” As with most things in Judaism, scholars have deliberated about this. The nun line was apparently already not there in the third century C.E., when it was discussed (recorded in the Talmud at Berakhot 4b, 21). Rabbi Yohanan rhetorically asks the question, and answers it by suggesting that the word for “fallen,” “nefilah,” would have begun with a nun, as in Amos 5:2 “Fallen is the Maiden of Israel, she will not rise again”), and because of its negativity the author left it out. We note that that word appears in the very next line - Somekh Adonai lekhol hanofelim, God supports the fallen - without superstition.
On the other hand, a nun verse actually appears in the Septuagint, which is the earliest Greek translation of what they term the Old Testament (translated beginning in the third century BCE), and in its Latin Vulgate translation (where this is numbered Psalm 144). The same verse also appears in the Syriac Peshitta (Christian translation from Hebrew into common language of Eastern Aramaic in the second century CE), and in similar form in the Dead Sea Scrolls. That line reads
.נֶאֱמָן י־ה בְכָל־דְּבָרָיו, וְחָסִיד בְּכָל מַעֲשָׂיו
Dead Sea Scrolls version is
.נאמן א־להים בדבריו, וחסיד בכל מעשיו
Which is rather similar to the tsade line:
.צַדְּיק י־ה בְכָל־דְּרָכָיו, וְחָסִיד בְּכָל מַעֲשָׂיו
However, other older texts did not have such a line, notably the Talmud, Aramaic Targum, and other Greek versions. Remember, we are talking about a span of a thousand years among all these translations.
Some folks mention W.H. Auden’s “Atlantis,” which is missing a line on purpose, claiming such poetic license could be this old. And there are those who feel an editor may have just thrown in the line upon noticing one was missing. That is just as plausible as someone leaving out the line when copying. Either way, scholars make the point that we can listen for the missing line and think of that which we do not want to mention, knowing that the Almighty will lift us up again anyway.