Edmund P. Hill, ed. and tr., Augustine,
Sermons III/8 (273-305A) on the Saints (Hyde Park: New City Press, 1994), p. 240, n. 6 (on Sermon 299.5-6):
From here to the beginning of section 6, as far as of whom I am the foremost, there is a hole in the only manuscript of this sermon, which belonged to the Abbey of Corbey. Only the first few words of the next ten lines survive. The reason, say the Maurists, is that "some good-for-nothing with too much time on his hands," quidam nebulo male feriatus, liked the illuminated initial letter of the manuscript on the reverse side so much, that he hacked it out.
See
Patrologia Latina, vol. 38, col. 1370, n. (b):
Haec lacuna contigit (ut saepe in elegantioribus manuscriptis) facinore nebulonis cujusdam male feriati, qui
litteram initialem hujus sermonis auro minioque depictam cultro praecidit.
Paris, Bibliothèque nationale, lat. 12202 and 13387, are 10th century manuscripts from Corbie containing sermons of Augustine. I don't know if either is the manuscript in question, and
I'm too lazy to look at them.