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venture (up)on someone or something
to come upon someone or something by chance. (Upon is formal and less commonly used than on. The entire expression is formal or stilted.) David ventured upon Fred, who was out looking for mushrooms. I ventured on a little shop on Maple Street that deals in old model trains.
venture onor venture upon
To come upon something by chance or fortune: The travelers ventured on a charming country inn. The explorers ventured upon a hidden cache of gold.