Another flash crash is, despite fragmentation, hugely unlikely. There is now an entire generation of traders having seen it once and standing ready to suck up stock the minute it starts dropping irrationally. In much the same way as terrorists will never again gain access to an airplane cockpit, because the model of taking control, landing it and demanding a ransom has been broken (by 9/11) and the customers (passengers) and pilots know there is no way they will allow terrorists to take control of the plane.