Synonyms:
unanticipated, unforeseen, unlooked-for, out_of_the_blue(p)
Meaning: not anticipated; "unanticipated and disconcerting lines of development"- H.W.Glidden; "unforeseen circumstances"; "a virtue unlooked-for in people so full of energy"; "like a bolt out of the blue"
Usage examples
The date of this period is one hundred and fifty millions of years hence, but unless some unforeseen catastrophe intervenes, it must assuredly come.
And most persons are naturally diverted from a proposed course of action by unusual, unforeseen obstacles, or by presentation of inducements to an action that is directly more agreeable.