How long before someone (a hacker no doubt) games the system to create a selling spiral, just waiting to take profit of the rebound.
If it´s well planned the one causing the dive is not going to be the one taking the profit (at least that it can be proved).
We need not wait for that to happen. Such shenanigans have been observed and documented already, such as in the book "Reminiscences of a Stock Operator." It was written in 1923, and worth reading today.
Well, it's been a common enough movie plot (cf. Casino Royale where the Antagonist engineers terrorist attacks on high-profile companies whilst short-selling their stocks).
It will probably happen when one of the stock exchanges says "we won't revert fraudulent trades anymore". Until then, I kind of doubt it.