Psalm 145 is an acrostic. In other words, each verse begins with a particular letter of the alphabet. Sometimes they spell out a name or a message, but in the Psalms they usually just follow the order of the letters of the alphabet. The latter is the case here. The Psalm, in the Masoretic text that is official in Judaism, has no verse beginning with "נ," "nun," the letter "n" of the Hebrew alphabet.
But there originally was such a verse, we know its meaning, and we can reconstruct it in Hebrew. The meaning is still present in the Septuagint (LXX), the Greek translation of the Hebrew Bible, dating from about 150 BCE. But by the time of St. Jerome (ca. 400 CE), the verse had disappeared from the Hebrew text.
When my friend originally asked me about this, I thought it was just a copyist's error, and no big deal. Here is the reconstruction of the verse in Hebrew:
נאמן יהוה בכל־דבריו וחסיד בּכל־מעשׂיו ׃
Here is the meaning:
The Lord is faithful in all his words,
and gracious in all his deeds.
This verse appeared in Hebrew Bibles at the time when the LXX translators did their job (ca. 150 BCE). It had, however, disappeared from current Hebrew texts of the Psalms by Jerome's time, 400 CE. Why?
What had happened between 150 BCE and 400 CE? Two things immediately come to mind: the destruction of the Second Temple, and the crushing of the Bar Kokhba revolt.
Is the omission of this verse from the Masoretic text that is the gold standard in Judaism to this day just an accident, a coincidence? Or had these words, in light of historical events, become too painful for the Jews to utter?
You decide.