The Tokyo Stock Exchange has admitted that its own system fault prevented an attempt to cancel a giant 27bn yen ($225m; £128m) share sale error.
The scandal has rocked the Japanese Stock Exchange
The mistake took place last week when a trader at Mizuho Securities mistyped a sale of shares in a firm called J-Com.
Instead of selling one share for 610,000 yen, he or she mistakenly sold 610,000 shares for 1 yen.
The Tokyo Stock Exchange admitted that a fault in its system meant Mizuho could not cancel the sale in time.
"We have found that malfunction of our own system was the cause for preventing Mizuho Securities from cancelling the sell order," the Tokyo Stock Exchange (TSE) said.
The TSE said Mizuho Securities had tried to withdraw the mistaken sale order shortly after noticing its error, but that its own system was too busy at the time to process the cancellation request.
Its admission comes as it continues an official probe into a scandal that has seen the Japanese government demand answers and the installation of a safety system to prevent anything similar from happening again.
The error has cost Mizuho 27bn yen so far, just below its entire 28.1bn yen profit for the financial year to March 2005.