As soon as Charles Comiskey saw what was coming, he turned off his machines.

It was still early in the New York trading day on Oct. 15 and investors were already pouring into U.S. government bonds as global financial markets from Asia to Europe buckled. Because yields were falling so fast, Comiskey, the head Treasury dealer at Bank of Nova Scotia, realized that he ran the risk of being stuck with losses or unwanted inventory if his computers automatically generated quotes to buy and sell with customers.

So for about half an hour, as yields on 10-year Treasuries tumbled below 2 percent in the biggest plummet in five years, he executed client orders individually over the phone.