“No matter what, expect the unexpected. And whenever possible BE the unexpected.”

As quoted in Alice James, Her Brothers — Her Journal (1934).

Source: The Phantom Tollbooth

Source: An Ideal Husband

Source: The Phantom Tollbooth

Fragment 18, as quoted in The Art and Thought of Heraclitus: An Edition of the Fragments (1981) edited by Charles H. Kahn, p. 105
Variants:
He who does not expect the unexpected will not find it out.
The Art and Thought of Heraclitus: An Edition of the Fragments (1981) edited by Charles H. Kahn, p. 129
He who does not expect the unexpected will not find it, since it is trackless and unexplored.
As quoted in Helen by Euripides, edited by William Allan (2008), p. 278
Unless you expect the unexpected, you will not find it, for it is hidden and thickly tangled.
Rendering ἐὰν μή "unless" is more English-friendly without being inaccurate. As for the last clause, the point is that you can neither find it nor navigate your way through it. The alpha-privatives suggest using similar metaphoric adjectives to keep the Greek 'feel.' (S. N. Jenks, 2014)
Numbered fragments

...y lo que me hace levantarme por las mañanas sigue siendo la espera de lo que está por llegar y no se anuncia, es la espera de lo inesperado, y no ceso de fantasear con lo que ha de venir.
Source: Todas las Almas [All Souls] (1989), p. 137

Source: The Charioteer

Source: All I Really Need to Know I Learned in Kindergarten