An inevitable mathematical problem faced by paytanim is the 22nd letter of the Hebrew Alphabet- Tav. Any poem using the classic 4-line-per-verse alphabetized structure is doomed to end at seven verses, at line 21- the letter shin. The tav is either omitted or deserving its own 4-line verse.
All the more so is the case of the classic Selihah, commonly found amogn those of Ta'anit Zibbur and Minhah of Yom Kippur- 'El na Refah' (Davidson 3875; Goldschmidt, Mahzor of Yom Kippur, 1970, p. 705). Here each of the seven verses conclude with the cry "'anenu…" mentioning Biblical heroes whose supplication for salvation was answered, the source of which is clearly the seven additional blessings supplemented to the prayer service for Public Fast that appear in Mishnah Ta'anit 2:4. The fourth line of each verse matches the corresponding blessing found in the Mishnah. This allows for only seven verses (21 lines, 21 letters), thus the end the Selihah at shin appears to be original, and so it appears in the medieval manuscripts (Mahzor Hildesheim p. 124; Modena, Archivio di Stato 184.1; Parma 3193 fol. 36a).
In the manuscript collection of the National Library, among the hundreds of loose manuscript folios and fragments found in the eight boxes labeled Ms. Heb. 8°1800, is a single paper leaf, containing only this Selichah. Although it starts and ends with lines from the standard Selihot service, the leaf appears to be organically individual, not having been detached from codex. These additional lines – in a different font size- are simply reminders of the place of where this piyyut is to be supplemented in the Selichot service. The paper bears a watermark similar to those found in the late 17th- early 18th cent. (Heawood, Watermarks, 1950, pls. 340-345). In this copy an additional eighth verse is supplied for the letter tav. The subject of the final "'anenu" here is Mordechai and Esther, as in the standard 'Anenu litany at the end of the Selihot (Goldschmidt, Selichot, 1965, p. 15). In the unlikely possibility that this is the original version, this verse was dropped because of the dissonance with the Mishnaic source. In the more likely scenario, in which the original ends at line shin, one must consider why this was added. The aesthetic asymmetry of a missing letter didn't seem to bother anyone before.
It's possible that this eighth verse was added for the usage of this piyyut in the Selihot service for the Fast of Esther.
Here's the additional verse:
תַעֲנֶה לְקורְאֶיךָ וְהַסְכֵת מִמְעונִים תִשְמַע שַוְעַת צועֲקֶיךָ שומֵעַ אֶל אֶבְיונִים תְרַחֵם בָנֶיךָ כְרַחֵם אָב עַל בָנִים עֲנֵנו כְמו שֶעָנִיתָ לְמָרְדְכָי וְאֶסְתֵר וְתָלוי עַל הָעֵץ חֲמִשִים הָאַב עִם בָנים:
And so too in two subsequent manuscripts (1690- Cincinnati HUC 774 p. 45a; 1790- Prague Jewish Museum 93 fol 2b). Note the relative flexibility of the length and content of this final verse, among the few text witnesses available- another indication that this is not an organic part of the poem. In Avodat Yisrael (Radleheim 1868 p. 595) the final line, possibly a result of apologetic self-censorship, reads:
עֲנֵנו כְשֶעָנִיתָ לְמָרְדְכָי וְאֶסְתֵר וְחַסְתָ עַל אָבות וּבָנִים