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by chance; by accident; accidentally; happen to (be, do); unexpectedly; casually
occasionally; once in a while; now and then
偶々立ち寄った店で良い品を見つけた。
I found something good at a shop I stopped by by chance.
偶々同じ電車に乗り合わせていた。
We happened to be on the same train.
偶々見かけた広告がきっかけで応募した。
I applied after seeing an ad by chance.