Horace, Epistles, vol.1, iii, lines 15 ff.

Context: the poet is gently mocking Celsus, who seems to be too imitative of already published poets.

quid mihi Celsus agit? monitus multumque monendus
privatas ut quaerat opes et tangere vitet
scripta Palatinus quaecumque recepit Apollo
ne, si forte suas repetitum venerit olim
grex avium plumas, moveat cornicula [i.e. Celsus] risum
furtivis nudatata coloribus.

Translation:

And what is my Celsus up to? Already [he’s been ] warned, and deserving further warning
that he should look to himself and avoid
the writings stored away in the Apollo Palatinus [library]
lest, if the flock of birds should come sometime
looking [for] their feathers, [and] the little crow provoke laughter
stripped [as he will be] of his stolen colors.

The idea seems to be that Celsus is too imitative, and that if he doesn’t watch out some clever wags will expose his borrowings, and subject Celsus to public mirth.

My problem is repetitum. My best guess is that this is an instance of the supine, here used to show the end or purpose of motion. The flock of birds, that is the earlier writers, may come back [venerit], in order to get back their feathers [suas … plumas]. But this solution came to me after I started typing this paragraph!

My best guess is that this is an instance of the supine, here used to show the end or purpose of motion.

Correct. You could have also figured this out that, if it weren’t supine, apart from the fact that it really couldn’t be anything else, the accusative suas […] plumas would be left hanging with no verb governing it.

many thanks anphph.

Why do you add “[and]”? moveat is the verb of the ne clause.

You may have understood this, but just to be clear, monitus in the first line is just the participle, continuing the syntax of the opening question. (You could translate e.g. “when he’s been warned ….”) Properly speaking the whole poem is a single sentence, and the question mark should go at the very end.
Lines 2-3 “(warned) to seek his own resources and to avoid touching whatever writings Palatine Apollo has received.” Perhaps you understood that too.

On the “[and]” I take your point mwh. The answer to the question, why the “[and]”, is hard for me to recover, now that, thanks to your critique, I see that the main-clause, subordinate-clause distinction should rule out “[and]”. But I was struggling with “repetitum” when I decided to type up the query. You and Hylander, in particular, often identify in my translations grammatical issues I wasn’t properly aware of when I posted a query on some other point. This is something for which I am most grateful.