Friday's trading volume soared to 54 percent above the previous day's level. That caused the main market index to spike up 6.5 percent before falling to end the day down 0.6 percent.
A brokerage, Everbright Securities, said it suffered an unspecified problem with a computerized trading system.
A government news agency, the China News Service, said Everbright sent 7 billion incorrect purchase orders for shares.
It said Everbright asked to have its trades canceled but a statement on the exchange website said any transactions that were completed would be cleared normally.
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