R. Shalom Carmy in "The Manufacture of Sulphurous Acid: Of Wisdom as a Catalyst in Torah Study" in Wisdom From All My Teachers, pp. 79-80:
Psalm 145 (widely known as "Ashrei" after the verses that preface it in liturgical usage) is an alphabetical acrostic. The verse corresponding to the letter nun is missing. The simple explanation is that the acrostic scheme is sometimes adhered to irregularly.  R. Yohanan (Berakhot 4b), however, proposed that the absent verse referred to nefilah (falling) and was omitted in order to avoid its negative associations. The following verse (15), which states that God supports those who fall, confirms the hypothesis that we have here a euphemistic elision. In this harmonious psalm, the calamity from which God rescues one is indicated by its noticeable omission.UPDATE: After posting this, I thought it would be nice to include a link to Auden's poem. Here it is. Looking at stanza 6, I wonder if Rabbi Carmy misinterpreted the pattern. I'm no poetry maven, but it seems to be consistent to me.
How seriously can one take this interpretation at the peshat (simple meaning) level? Is it likely that the author of a twenty-two-line poem would deliberately breach the poetic form of the composition in order to make a subtle point that is likely to be lost on the casual reader? Is R. Yohanan not reading an idea into the text that has no purchase on the text?
This issue was far from my mind the day I read W.H. Auden's "Atlantis." The poem, comprising seven twelve-line stanzas, which exhibit a complicated pattern of rhyme and meter, describes the effort and resourcefulness required to reach the mythical islan of Atlantis. The voyage culminates in a scene where the traveler, having overcome many ordeals, collapses: "With all Atlantis shining/ Below you yet you cannot/ Descend." At this precise point in the poem, the rigid pattern is violated: line 7 of stanza 6 does not exist. The explanation seems obvious: the poet's "failure" to fully satisfy the complicated technical feat he has undertaken parallels the failure of the poem's protagonist to consummate his journey. The intertwining of form and content in the work of a twentieth-century master craftsman renders more persuasive the notion of a similar phenomenon in the psalm.
 See Ps. 25:2, 5, 17, 34:6, 9-10. The 11Qpsalms Scroll supplies the missing verse (likewise the Septuagint). This verse, however, is close to v. 17, except for the initial substitution of ne'eman for tzaddik and the replacement of the Tetragrammaton with Elokim. Hence this version is presumably a scribal solution to the problem of the absent verse, rather than an original alternative. See also Amos Hakham, Da'at Mikra: Tehillim, vol. 2 (Jerusalem, 1984) 578f. and note 23.
FURTHER UPDATE: Menachem Butler asked Rabbi Carmy and reports the following: The line "Even to have been allowed" (found on the online version of this poem) does not appear in the print version of Auden’s Atlantis. Thus, it is as Rabbi Carmy wrote, "line 7 of stanza 6 does not exist."