The boss of Nasdaq has blamed staff arrogance and overconfidence for Facebook's (NASDAQ: FB) botched initial public offering last month.

CEO Robert Greifeld told a conference of directors at Stanford University's Law School that the exchange had tested its computer systems before the May 18 first trades but failed to take into account the volume of canceled orders in the run-up to the stock debut, the Wall Street Journal reported.

A 30-minute delay at the start of Facebook's trading and problems completing and canceling orders once the sale got under way caused losses of more than $500 million, according to investors and financial firms.

Facebook shares Monday fell 54 cents to $32.51 in early trading. They're down 14 percent from the $38 IPO price.

Testing didn't account for the increasing volume at which cancellations can come in, Greifeld said.

There was not enough of a check and balance, he said. We did not have enough business judgment in the process.

Defending Nasdaq's reputation, he said the company's ordering system had handled 480 IPOs in the past five years, but that an unexpectedly high number of cancellations in the hours leading up to the Facebook offering had caused the problems.

He added that had Nasdaq, owned by Nasdaq OMX Group (Nasdaq: NDAQ), known it would take until mid-afternoon to confirm all the Facebook trades, the outcome would have been different.

Nasdaq shares fell 51 cents to $21.26 on Monday. They're down 4.5 percent since the Facebook IPO.