fluke
FLOOKnounMeaning a lucky or unlikely chance happening
Memory hook it FLEW in by LUCK. Pip tosses a paper ball over his shoulder without even looking, and it flies straight into the trash can across the room; a lucky shot like that is a fluke.
Her first basket from half court was a fluke; she missed the next twenty.
In context Some called the underdogs’ win a fluke, but when the team won its next three games as well, the doubters went quiet.
Similar luck, happy accident, stroke of luck, lucky break
Opposite sure thing, certainty