“A complex bet in the foreign-exchange market backfired on Goldman Sachs Group Inc. during the third quarter, people familiar with the matter said.

“The previously undisclosed trading mishap played a part in a revenue slump that prompted senior executives to defend the firm’s trading strategy.

“Goldman last month said revenue in its currency-trading unit fell sharply in the third quarter from the second.

“Within that group, the firm’s foreign-exchange options desk posted a net loss during the period partly because a bet on the value of the dollar versus the Japanese yen went wrong, the people said.“

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