The most plausible explanation, however, is that traders were gaming the system. The Hong Kong exchange is not alone in running a 10-minute auction every afternoon to settle outstanding trades: almost every other major market does this too, to calculate a price that will satisfy as many orders as possible. But unlike others, Hong Kong has no system of random closing: the auction ends every day at 4:08. That means that market participants have an incentive to time their bids. In this case, the downswing in HSBC caused by an order seconds before the deadline knocked about 212 points off the HSI. An issuer of index options callable that day might have been willing to wear a big loss on the shares.

Hong Kong's auction regime is less than a year old, so it can be excused a few teething-problems: London ran into plenty of difficulties nine years ago. Proposed tweaks from the HKEx may limit the damage, but do nothing to resolve that basic flaw. The market, and its traumatised retail shareholders, deserve better.