True, but at the same time, we're talking about billions of dollars of real money. In clear self interest, all those institutions will have built obvious checks

True, but at the same time, we're talking about billions of dollars of real money. In clear self interest, all those institutions will have built obvious checks in their system. Losing several billions of dollars in seconds is something they want to avoid. Indeed, the fact that the flash crash was the definition of something with a vast number of transactions, huge price moves, and happened in the blink of an eye but still was relatively minor and almost inconsequential shows the reliance of the stock market.