Here are my nominations:
1. In a Greek text: In Volume I of R. D. Dawe’s Teubner Sophocles (1975), the first word of Oedipus Tyrannus is misspelled. The fact that it’s a one-letter word is particularly impressive:
τέκνα Κάδμου τοῦ πάλαι νέα τροφή
This was corrected to Ὦ in the second edition (1984).
2. In a Latin text: In D. R. Shackleton Bailey’s Teubner text of Horace, the last word of the Horatian corpus is misspelled, turning Horace’s leech who will not let go until full of blood into a bird, specifically a swallow:
non missura cutem nisi plena cruoris hirundo
I’m told this was corrected to hirudo in later printings.
3. In a secondary source: In the Cambridge History of Classical Literature, II: Latin Literature (253), one of the most famous sentences in the Latin language is botched:
Quousque tandem abutere, Catilina, patientiam nostram?
It looks like some officious proofreader ‘corrected’ the case of the last two words, forgetting the rule about ‘utor, fruor, fungor, vescor, potior, and their compounds’ and assuming that abutor takes the accusative like a normal verb.