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by chance; by accident; accidentally; happen to (be, do); unexpectedly; casually
occasionally; once in a while; now and then
道を歩いていて偶然昔の友人に出会った。
I was walking down the street and happened to meet an old friend.
たまたま空いていた席に座れた。
I happened to get an empty seat.
たまたま見かけた店で素敵な服を見つけた。
I found nice clothes at a shop I happened to spot.