One simple way to prevent fluctuations like those of last Thursday would be for stock exchanges to prohibit orders to buy or sell at the market.
That wouldn’t mean prohibiting orders that act a lot like market orders. People could still be allowed to place an order to sell at a limit of a penny. But having an explicit limit price would discourage people from entering orders that under rare conditions end up being executed at a price 99 percent lower than expected.
It wouldn’t even require that people take the time to type in a limit price. Systems could be designed to have a pseudo-market order that behaves a lot like existing market orders, but which has a default limit price that is, say, 5 percent worse than the last reported price.
However, it’s not obvious to me that those of us who didn’t sell at ridiculously low prices should want any changes in the system. Moderate amounts of money were transferred mainly from people who mistakenly thought they were sophisticated traders to people who actually were. People who are aware that they are amateurs rarely react fast enough to declines to have done anything before prices recovered. The decline looked like it was primarily the result of stop-loss strategies, and it’s hard to implement those without at least superficially imitating an expert investor.