The prayer Ashrei, which we recite three times a day, is an acrostic consisting of all the letters of the Hebrew alphabet except for nun (נ). Why is the nun omitted?
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The g'mara in B'rachos (4B) explains that 'נ' represents downfall [of the nation] and is therefore encompassed in the positive context of the putative next pasuk, which states that "God supports all of the fallen".
See this post on On The Main Line about a nun verse in Ashrei from antiquity, and whether is was original: