The greatest of sages can commit one mistake, but not two; he may fall into error, but he doesn’t lie down and make his home there.

[En un descuido puede caer el mayor sabio, pero en dos no; y de paso, que no de asiento.]

Baltasar Gracián y Morales (1601-1658) Spanish Jesuit priest, writer, philosopher

The Art of Worldly Wisdom [Oráculo Manual y Arte de Prudencia], § 214 (1647) [tr. Maurer (1992)]
    (Source)

(Source (Spanish)). Alternate translations:

The wisest man may very well fail once, but not twice; transiently, and by inadvertency, but not deliberately.
[Flesher ed. (1685)]

A wise man may make one slip but never two, and that only in running, not while standing still.
[tr. Jacobs (1892)]

The wisest of men may slip once, but not twice, and that only by chance, and not by design.
[tr. Fischer (1937)]