Erasmus, Adagia 30:
The same thought is put forth by others: Ῥεχθὲν δε τε νήπιος ἔγνω, that is, ‘The fool understands the matter once it’s done.’ It has, however, been taken from Homer, who has taken up this thought in many places, as in Μήτε ἀντίος ἵστασ᾽ ἐμοῖο, Πρίν τι κακὸν παθέειν· ῥεχθὲν δέ τε νήπιος ἔγνω, that is, Beware of coming up against me, before you take up some harm; for even the stupid person understands the deed once it’s done. Euripides has alluded to this in his Bacchae: Κακοῦ γὰρ ἐγγὺς ὢν ἐμάνθανεν, that is, For he learned, being close to the misfortune; this was said of Pentheus, who learned too late and not without danger to himself to revere Bacchus. Not entirely dissimilar to this is that senarius renowned among the Greek sententiae: Ἡ δὲ μετάνοια γίγνετ᾽ ἀνθρώποις κρίσης, that is, Then people judge, when they already have regret. This expression of Vergil has the same bearing: Having been warned, learn justice and not to spurn the gods. Similarly, the thought of Demosthenes: I do not purchase regret at such a price. And so, in the most elegant way, Fabius (in Livy) calls the outcome the teacher of the fool, saying, Nor would the outcome, which is the teacher of the fool, teach this. Pliny, in his Panegyric which he spoke to Trajan, calls prudence of this sort fruitless and wretched, saying Terror, and fear, and that wretched prudence made out of dangers warned us to turn our eyes, our ears, our minds away from the republic (though there was, however, no republic at all).
FACTVM STVLTVS COGNOSCIT
Idem aliter effertur ab aliis: Ῥεχθὲν δε τε νήπιος ἔγνω, id est• Rem peractam stultus intellexit•. Sumptum est autem ex Homero, qui pluribus locis hanc vsurpauit sententiam. Vt in Iliados Μήτε ἀντίος ἵστασ᾽ ἐμοῖο, Πρίν τι κακὸν παθέειν· ῥεχθὲν δέ τε νήπιος ἔγνω, id est Mihi obuius ire caueto, prius quam Noxae aliquid capias; nam factum nouit et excors. Huc allusit Euripides in Bacchis: Κακοῦ γὰρ ἐγγὺς ὢν ἐμάνθανεν, id est Nam didicit affinis malo, de Pentheo, qui sero nec nisi sua pernicie doctus coepit reuereri Bacchum. Neque huic diuersum est, quod admonet senarius ille inter Graecanicas sententias celebris: Ἡ δὲ μετάνοια γίγνετ᾽ ἀνθρώποις κρίσης, id est Tum iudicant homines, vbi iam poenitet. Eodem pertinet Vergilianum illud: Discite iustitiam moniti et non temnere diuos. Item illud Demosthenicum: Non emo tanti poenitere. Vnde perquam eleganter Fabius apud Titum Liuium euentum stultorum magistrum appellat Nec euentus doceat hoc, inquiens, qui stultorum magister est, sed ratio. Plinius in Panegyrico, quem Traiano dixit, huiusmodi seram et infrugiferam prudentiam miseram vocat. Terror, inquit, et metus et misera illa ex periculis facta prudentia monebat, vt a republica (erat autem omnino nulla respublica) oculos, aures, animos auerteremus.