All men make mistakes; but it is fools who persist in them.

[Cuiusvis hominis est errare, nullius nisi insipientis in errore perseverare.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher

Philippics [Philippicae; Antonian Orations], No. 12, ch. 2 / sec. 5 (12.2/12.5) (43-03 BC) [tr. @sentantiq (2012)]
    (Source)

(Source (Latin)). Other translations:

Every man may err, but no man who is not a fool may persist in error.
[ed. Harbottle (1897)]

Any man is liable to a mistake; but no one but a downright fool will persist in error.
[tr. Yonge (1903)]

Every man is liable to err; it is the part only of a fool to persevere in error.
[tr. Ker (Loeb) (1926)]

Any man is liable to err, only a fool persists in error.
[ed. Guterman (1966)]

Of any man at all it is to err, to persist in error is of none except unthinking.
[tr. Wiseman]