Apple shares were briefly halted Friday morning on a single-stock circuit breaker after the price dropped 9.4% to US$542.80.

The shares were halted at 10:57 a.m. (Eastern); trading resumed at 11:02 a.m. So what happened? “StreetInsider.com” (http://macte.ch/8ubdI) says it was due to an errant trade.

According to John Melloy at “CNBC,” one trade at $542.80 of 100 shares crossed at 10:57:36 from the BATS exchange. Coincidentally, BATS is scheduled to come public today, notes “StreetInsider.com.”

“It appears a bad trade triggered the halt,” adds The Wall Street Journal.” “Stocks are haltedĀ if they swing more than 10% in a span of five minutes. This allows market participants time to regroup and to prevent further erroneous trades from undermining the market.”