Most (theoretically all) automated trading is going through some sort of risk checking software to prevent it from doing just that. The broker dealers are only

Most (theoretically all) automated trading is going through some sort of risk checking software to prevent it from doing just that. The broker dealers are only allowed by regulation (enacted after the flash crash) to allow the algorithm guys to respond so aggressively (the limit is usually an internally set one but you gotta be within it or the feds get on your case) to changes in the market.