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Synonyms for Unexpected (same or very similar meaning)

WordNet sense 1 (made necessary by an unexpected situation or emergency):
forced

WordNet sense 2 (not scheduled or not on a regular schedule):
unscheduled

WordNet sense 3 (not anticipated):
unforeseen, unlooked-for

WordNet sense 4 (happening without warning or in a short space of time):
sudden


From the ODE community, based on WordNetadd/edit