Another interesting anecdote -- most HFT groups shut off during the flash crash because they were running strategies that were fit on normal market conditions a

Another interesting anecdote -- most HFT groups shut off during the flash crash because they were running strategies that were fit on normal market conditions and really had no idea how their algorithm would perform in flash crash conditions. But not all HFTs turned off, and I know of at last two groups that kept running throughout the whole thing (trading equities, futures, currencies) and they made a phenomenal amount of money. Many traders I've talked to severely regretted turning off their strategies, and I've heard the argument that if such an event happens again, we're likely to see a lot more groups continue to run everything instead of completely turning off.