Ξύλον ἀγκύλον οὐδέποτ' ὀρθόν → a bent board is never straight
— Diogenianus (2nd cent.), Proverb 6.92. The grammarian/paroemiographer goes on to explain: ὅτι δυσχερὲς ἐκ φαύλων ἀγαθοὺς ἀπεργάσασθαι, "because it's hard to make good people out of bad."
Latin: lignum curvum nunquam rectum
German: krummes Holz wird niemals gerade