Well, they have been, it's been testing for months — but there are still some conditions that apparently don't get created until you're in a real environment, or are difficult to simulate. (Read more: Panic Selling? Investors Are Dumping Dividend Stocks)
Second, trading continued in these stocks elsewhere. Trading was halted on the NYSE floor, but trading continued as normal on Arca, the NYSE's electronic platform, and on other exchanges. It was like dropping a rock in a stream: the water just flowed around it.
This is very different than what would have happened 10 years ago, when the NYSE floor controlled 80 percent of its own volume. Halting trading then would have been a much bigger issue. (Read more: Marc Faber: Prepare for a Massive Market Meltdown)
Upgrading of systems is a critical job for all the exchanges. They have to upgrade hardware and software to increase speed and capacity and make it easier for all these systems to interact with each other and the outside world. The problem remains testing them in a real-world environment.
—By CNBC's Bob Pisani
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