Sarao says he used the program to make thousands of bogus orders for E-mini S&P 500 futures contracts (including 85 sell orders on the day of the crash), creating an illusion of supply that fooled others into trading when they otherwise wouldn't. When that happened, he'd make real trades to capitalize on the artificial values. Assistant Attorney General Leslie Caldwell claims that Sarao not only hurt "countless" traders with these money grabs, but risked ruining the "integrity" of American stock markets in the process.

Sentencing has yet to take place. With that said, Sarao's actions might have headed off some attempts to game the system with rapid-fire, software-based trading. While these sorts of incidents still happen, the "Flash Crash" raised awareness of the problem and helped foster concepts like circuit breakers, which automatically halt trading when there's a sudden and dramatic spike in activity. The guilty plea may close a key chapter in financial history, but it's also a reminder that safeguards are still important for preventing digital scams.