Is this your crossword clue?
"unplanned" is also used as a crossword clue. If that's what you saw, the answer is often spontaneous (11 letters).
Definitions
not done with purpose or intent
Word origin
From un- + planned.
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer1 curated clues
01“Not scheduled”9 letters
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U________