Trading in shares of Progress , a Raleigh, N.C., utility operator, were halted for five minutes Monday afternoon after the stock briefly plunged 90% for no apparent reason.

The stock dropped from around $44.50 to $4.57 just before 1 p.m. EDT before resuming trading above $44 a few minutes later. The New York Stock Exchange, where Progress shares are listed, said the troublesome trade took place on the Nasdaq.

Update: Nasdaq says “an inaccurate limit price was entered by a trading firm. No trades were routed to other market centers. Approximately 57 trades have been cancelled.”

The plunge paused trading for five minutes thanks to the single-stock circuit breaker program put in place after the May 6 flash crash, which saw the stock market tumble almost a thousand points in the space of 15 minutes.

Regulators have been probing that incident and are expected to make some rule changes in response later this year. Some skeptics say evidence points to abuses by the high-frequency traders whose frenetic computerized paper shuffling accounts for a rising share of market activity.

Monday’s trading pause is the fourth in the past month. Trading in Seagate was halted briefly last week, flowing similar incidents in Nucor on Sept. 14 and in Intel Aug. 27.

Whatever their cause, the frequent market outages only feed the sense that the entire market is either a casino rigged by the money never sleeps crowd or a house of cards on the verge of collapse. Neither view, it seems safe to say, is apt to restore investors’ dwindling confidence.