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by chance; by accident; accidentally; happen to (be, do); unexpectedly; casually
occasionally; once in a while; now and then
たまたま通りかかった人に道を教えてもらった。
A passerby who happened to be there gave me directions.
たまたま財布を忘れていて支払えなかった。
I happened to have forgotten my wallet and couldn't pay.
たまたま空いていた席に座ることができた。
I was able to sit in a seat that happened to be empty.