Trouble began Thursday morning, when Mizuho Securities tried to sell 610,000 shares at 1 yen (less than a penny) apiece in a job recruiting firm called J-Com Co., which was having its public debut on the exchange.

It had actually intended to sell 1 share at 610,000 yen ($5,041).

Worse still, the number of shares in Mizuho's order was 41 times that J-Com's true outstanding amount, but the Tokyo Stock Exchange processed the order anyway.

Mizuho says it tried to cancel the order three times, but the exchange said it doesn't cancel transactions even if they are executed on erroneous orders.

By the end of the day, Mizuho Securities _ a division of the nation's second-largest bank, Mizuho Financial Group, Inc. _ had lost at least 27 billion yen. That total could escalate, however, as more trades settle, Mizuho Securities spokesman Hideki Sakuma said Friday, adding that the mishap was sparked by human error.