The unleashing of ¥67.78 trillion ($617 billion) of mistaken stock orders in Japan is reviving concern about the accountability of brokers overseeing trades in the world's second-biggest equity market.

While the orders were canceled before being executed, averting losses for whoever placed them, they highlight the potential for catastrophe anytime someone gets details wrong with instructions to buy or sell securities. One of the biggest U.S. market makers, Knight Capital Group Inc., almost went bankrupt in 2012 after its computers bombarded U.S. exchanges with erroneous orders that couldn't be undone.

The Knight incident spurred a yearlong review by U.S. regulators into procedures used by brokerages to prevent computer and software errors from roiling markets. Though the sheer size of Wednesday's orders in Japan may have kept them from being carried out, executives at trading firms said they were concerned about how they reached the market.