“Truly nothing is to be expected but the unexpected.”
As quoted in Alice James, Her Brothers — Her Journal (1934).
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Alice James 2
American diarist 1848–1892Related quotes

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

“He who does not expect will not find out the unexpected, for it is trackless and unexplored.”
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

“A thing long expected takes the form of the unexpected when at last it comes.”